Add flag that represents the traversal is only for animation-only restyle.

We will set the flag when the root node of the traversal has
has-animation-only-dirty-descendants flag or has animation restyle hints.
Also we will use this flag to detect whether we need to trigger CSS transitions
or not.

components/layout_thread/lib.rs

@@ -520,6 +520,7 @@ impl LayoutThread {
                 local_context_creation_data: Mutex::new(thread_local_style_context_creation_data),
                 timer: self.timer.clone(),
                 quirks_mode: self.quirks_mode.unwrap(),
+                animation_only_restyle: false,
             },
             image_cache_thread: Mutex::new(self.image_cache_thread.clone()),
             font_cache_thread: Mutex::new(self.font_cache_thread.clone()),

components/style/context.rs

@@ -87,6 +87,9 @@ pub struct SharedStyleContext<'a> {
 
     /// The QuirksMode state which the document needs to be rendered with
     pub quirks_mode: QuirksMode,
+
+    /// True if the traversal is processing only animation restyles.
+    pub animation_only_restyle: bool,
 }
 
 impl<'a> SharedStyleContext<'a> {

components/style/dom.rs

@@ -372,6 +372,16 @@ pub trait TElement : PartialEq + Debug + Sized + Copy + Clone + ElementExt + Pre
 
     /// Returns true if the element has a CSS animation.
     fn has_css_animations(&self, _pseudo: Option<&PseudoElement>) -> bool;
+
+    /// Returns true if the element has animation restyle hints.
+    fn has_animation_restyle_hints(&self) -> bool {
+        let data = match self.borrow_data() {
+            Some(d) => d,
+            None => return false,
+        };
+        return data.get_restyle()
+                   .map_or(false, |r| r.hint.has_animation_hint());
+    }
 }
 
 /// TNode and TElement aren't Send because we want to be careful and explicit