Bug 1355343: Take all the snapshots into account. r=bholley

I've chosen this approach mainly because there's no other good way to guarantee
the model is correct than holding the snapshots alive until a style refresh.

What I tried before this (storing them in a sort of "immutable element data") is
a pain, since we call into style from the frame constructor and other content
notifications, which makes keeping track of which snapshots should be cleared an
which shouldn't an insane task.

Ideally we'd have a single entry-point for style, but that's not the case right
now, and changing that requires pretty non-trivial changes to the frame
constructor.

MozReview-Commit-ID: FF1KWZv2iBM
Signed-off-by: Emilio Cobos Álvarez <emilio@crisal.io>

components/style/servo/selector_parser.rs

@@ -9,7 +9,9 @@
 use {Atom, Prefix, Namespace, LocalName};
 use attr::{AttrIdentifier, AttrValue};
 use cssparser::{Parser as CssParser, ToCss, serialize_identifier};
+use dom::{OpaqueNode, TElement, TNode};
 use element_state::ElementState;
+use fnv::FnvHashMap;
 use restyle_hints::ElementSnapshot;
 use selector_parser::{ElementExt, PseudoElementCascadeType, SelectorParser};
 use selectors::{Element, MatchAttrGeneric};
@@ -20,6 +22,7 @@ use std::borrow::Cow;
 use std::fmt;
 use std::fmt::Debug;
 use std::mem;
+use std::ops::{Deref, DerefMut};
 
 /// A pseudo-element, both public and private.
 ///
@@ -448,6 +451,36 @@ impl SelectorImpl {
     }
 }
 
+/// A map from elements to snapshots for the Servo style back-end.
+#[derive(Debug)]
+pub struct SnapshotMap(FnvHashMap<OpaqueNode, ServoElementSnapshot>);
+
+impl SnapshotMap {
+    /// Create a new empty `SnapshotMap`.
+    pub fn new() -> Self {
+        SnapshotMap(FnvHashMap::default())
+    }
+
+    /// Get a snapshot given an element.
+    pub fn get<T: TElement>(&self, el: &T) -> Option<&ServoElementSnapshot> {
+        self.0.get(&el.as_node().opaque())
+    }
+}
+
+impl Deref for SnapshotMap {
+    type Target = FnvHashMap<OpaqueNode, ServoElementSnapshot>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl DerefMut for SnapshotMap {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}
+
 /// Servo's version of an element snapshot.
 #[derive(Debug)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]