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/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> {