Add an author_style_disabled flag to stylist.update, and associated structs.

MozReview-Commit-ID: FiXyEN4xVnU

components/style/stylist.rs

@@ -78,6 +78,9 @@ pub struct Stylist {
     /// If true, the quirks-mode stylesheet is applied.
     quirks_mode: bool,
 
+    /// If true, authored styles are ignored.
+    author_style_disabled: bool,
+
     /// If true, the device has changed, and the stylist needs to be updated.
     is_device_dirty: bool,
 
@@ -134,6 +137,10 @@ pub struct ExtraStyleData<'a> {
     #[cfg(feature = "gecko")]
     pub font_faces: &'a mut Vec<(Arc<Locked<FontFaceRule>>, Origin)>,
 
+    /// A parameter to change a setting to ignore author styles during update.
+    /// A None value indicates that update should use existing settings.
+    pub author_style_disabled: Option<bool>,
+
     #[allow(missing_docs)]
     #[cfg(feature = "servo")]
     pub marker: PhantomData<&'a usize>,
@@ -167,6 +174,7 @@ impl Stylist {
             device: Arc::new(device),
             is_device_dirty: true,
             quirks_mode: false,
+            author_style_disabled: false,
 
             element_map: PerPseudoElementSelectorMap::new(),
             pseudos_map: Default::default(),
@@ -274,7 +282,16 @@ impl Stylist {
             }
         }
 
-        for ref stylesheet in doc_stylesheets.iter() {
+        // Absorb changes to author_style_disabled, if supplied.
+        if let Some(author_style_disabled) = extra_data.author_style_disabled {
+            self.author_style_disabled = author_style_disabled;
+        }
+
+        // Only use author stylesheets if author styles are enabled.
+        let author_style_enabled = !self.author_style_disabled;
+        let sheets_to_add = doc_stylesheets.iter().filter(
+            |&s| author_style_enabled || s.origin != Origin::Author);
+        for ref stylesheet in sheets_to_add {
             self.add_stylesheet(stylesheet, guards.author, extra_data);
         }