Bug 1341372 - Part 6: Trigger transitions.

1. We need to call get_after_change_style, which is the computed styles
   without transition rules, while process_animations.
2. If we have after-change style, we may replace the new computed values with
   after-change style, according to whether we really need to update
   transitions.
3. There are some cases we don't update transitions, so we need to early
   return. might_needs_transitions_update() will check it first and it
   will filter out most common cases.
4. needs_transitions_update() will check each property and existing running
   transitions to make sure we really don't need to update transitions.
   The logic of this function is similar with that of
   nsTransitionManager::DoUpdateTransitions().

MozReview-Commit-ID: 2ccdPjgrxKz

components/style/gecko_bindings/bindings.rs

@@ -653,6 +653,25 @@ extern "C" {
                                           aPseudoTagOrNull: *mut nsIAtom)
      -> bool;
 }
+extern "C" {
+    pub fn Gecko_ElementTransitions_Length(aElement: RawGeckoElementBorrowed,
+                                           aPseudoTagOrNull: *mut nsIAtom)
+     -> usize;
+}
+extern "C" {
+    pub fn Gecko_ElementTransitions_PropertyAt(aElement:
+                                                   RawGeckoElementBorrowed,
+                                               aPseudoTagOrNull: *mut nsIAtom,
+                                               aIndex: usize)
+     -> nsCSSPropertyID;
+}
+extern "C" {
+    pub fn Gecko_ElementTransitions_EndValueAt(aElement:
+                                                   RawGeckoElementBorrowed,
+                                               aPseudoTagOrNull: *mut nsIAtom,
+                                               aIndex: usize)
+     -> RawServoAnimationValueBorrowedOrNull;
+}
 extern "C" {
     pub fn Gecko_GetProgressFromComputedTiming(aComputedTiming:
                                                    RawGeckoComputedTimingBorrowed)