Move Stylesheet loading and ownership from the layout task into HTML elements

Stylesheets for `HTMLLinkElement`s are now loaded by the resource task, triggered by the element in question. Stylesheets are owned by the elements they're associated with, which can be `HTMLStyleElement`, `HTMLLinkElement`, and `HTMLMetaElement` (for `<meta name="viewport">).

Additionally, the quirks mode stylesheet (just as the user and user agent stylesheets a couple of commits ago), is implemented as a lazy static, loaded once per process and shared between all documents.

This all has various nice consequences:
 - Stylesheet loading becomes a non-blocking operation.
 - Stylesheets are removed when the element they're associated with is removed from the document.
 - It'll be possible to implement the CSSOM APIs that require direct access to the stylesheets (i.e., ~ all of them).
 - Various subtle correctness issues are fixed.

One piece of interesting follow-up work would be to move parsing of external stylesheets to the resource task, too. Right now, it happens in the link element once loading is complete, so blocks the script task. Moving it to the resource task would probably be fairly straight-forward as it doesn't require access to any external state.

components/script/dom/document.rs

@@ -48,7 +48,10 @@ use dom::htmlheadelement::HTMLHeadElement;
 use dom::htmlhtmlelement::HTMLHtmlElement;
 use dom::htmliframeelement::{self, HTMLIFrameElement};
 use dom::htmlimageelement::HTMLImageElement;
+use dom::htmllinkelement::HTMLLinkElement;
+use dom::htmlmetaelement::HTMLMetaElement;
 use dom::htmlscriptelement::HTMLScriptElement;
+use dom::htmlstyleelement::HTMLStyleElement;
 use dom::htmltitleelement::HTMLTitleElement;
 use dom::keyboardevent::KeyboardEvent;
 use dom::location::Location;
@@ -96,8 +99,10 @@ use std::default::Default;
 use std::iter::FromIterator;
 use std::ptr;
 use std::rc::Rc;
+use std::sync::Arc;
 use std::sync::mpsc::channel;
 use string_cache::{Atom, QualName};
+use style::stylesheets::Stylesheet;
 use time;
 use url::Url;
 use util::str::{DOMString, split_html_space_chars, str_join};
@@ -135,6 +140,10 @@ pub struct Document {
     scripts: MutNullableHeap<JS<HTMLCollection>>,
     anchors: MutNullableHeap<JS<HTMLCollection>>,
     applets: MutNullableHeap<JS<HTMLCollection>>,
+    /// List of stylesheets associated with nodes in this document. |None| if the list needs to be refreshed.
+    stylesheets: DOMRefCell<Option<Vec<Arc<Stylesheet>>>>,
+    /// Whether the list of stylesheets has changed since the last reflow was triggered.
+    stylesheets_changed_since_reflow: Cell<bool>,
     ready_state: Cell<DocumentReadyState>,
     /// Whether the DOMContentLoaded event has already been dispatched.
     domcontentloaded_dispatched: Cell<bool>,
@@ -983,6 +992,21 @@ impl Document {
         count_cell.set(count_cell.get() - 1);
     }
 
+    pub fn invalidate_stylesheets(&self) {
+        self.stylesheets_changed_since_reflow.set(true);
+        *self.stylesheets.borrow_mut() = None;
+        // Mark the document element dirty so a reflow will be performed.
+        self.get_html_element().map(|root| {
+            root.upcast::<Node>().dirty(NodeDamage::NodeStyleDamaged);
+        });
+    }
+
+    pub fn get_and_reset_stylesheets_changed_since_reflow(&self) -> bool {
+        let changed = self.stylesheets_changed_since_reflow.get();
+        self.stylesheets_changed_since_reflow.set(false);
+        changed
+    }
+
     pub fn set_pending_parsing_blocking_script(&self, script: Option<&HTMLScriptElement>) {
         assert!(self.get_pending_parsing_blocking_script().is_none() || script.is_none());
         self.pending_parsing_blocking_script.set(script);
@@ -1100,6 +1124,13 @@ impl Document {
             if parser.is_suspended() {
                 parser.resume();
             }
+        } else if self.reflow_timeout.get().is_none() {
+            // If we don't have a parser, and the reflow timer has been reset, explicitly
+            // trigger a reflow.
+            if let LoadType::Stylesheet(_) = load {
+                self.window().reflow(ReflowGoal::ForDisplay, ReflowQueryType::NoQuery,
+                                     ReflowReason::StylesheetLoaded);
+            }
         }
 
         let loader = self.loader.borrow();
@@ -1304,6 +1335,8 @@ impl Document {
             scripts: Default::default(),
             anchors: Default::default(),
             applets: Default::default(),
+            stylesheets: DOMRefCell::new(None),
+            stylesheets_changed_since_reflow: Cell::new(false),
             ready_state: Cell::new(ready_state),
             domcontentloaded_dispatched: Cell::new(domcontentloaded_dispatched),
             possibly_focused: Default::default(),
@@ -1369,6 +1402,31 @@ impl Document {
         self.GetDocumentElement().and_then(Root::downcast)
     }
 
+    /// Returns the list of stylesheets associated with nodes in the document.
+    pub fn stylesheets(&self) -> Ref<Vec<Arc<Stylesheet>>> {
+        {
+            let mut stylesheets = self.stylesheets.borrow_mut();
+            if stylesheets.is_none() {
+                let new_stylesheets: Vec<Arc<Stylesheet>> = self.upcast::<Node>()
+                    .traverse_preorder()
+                    .filter_map(|node| {
+                        if let Some(node) = node.downcast::<HTMLStyleElement>() {
+                            node.get_stylesheet()
+                        } else if let Some(node) = node.downcast::<HTMLLinkElement>() {
+                            node.get_stylesheet()
+                        } else if let Some(node) = node.downcast::<HTMLMetaElement>() {
+                            node.get_stylesheet()
+                        } else {
+                            None
+                        }
+                    })
+                    .collect();
+                *stylesheets = Some(new_stylesheets);
+            };
+        }
+        Ref::map(self.stylesheets.borrow(), |t| t.as_ref().unwrap())
+    }
+
     /// https://html.spec.whatwg.org/multipage/#appropriate-template-contents-owner-document
     pub fn appropriate_template_contents_owner_document(&self) -> Root<Document> {
         self.appropriate_template_contents_owner_document.or_init(|| {