Bug 1171817 part 1 - Cancel animations when destroying the property holding them; r=dbaron

Prior to this patch we cancel animations in AnimationCollection::Destroy but
this is not called automatically when the property holding the collection is
destroyed via its destructor. When an element is unbound from the tree we
destroy its animation properties but don't call AnimationCollection::Destroy.
This means, that in such circumstances:

* We won't create animation mutation records for the removed animations
* Once we start registering animations with a timeline they won't have a
  chance to remove themselves from the timeline (meaning
  document.timeline.getAnimations()) will keep returning them
* Once we go to implement the animationcancel and transitioncancel events we
  won't fire them in this case (assuming we implement the queueing/dispatch of
  those events as part of the cancel code)

This patch addresses this by moving the call to cancel each animations to the
property destructor for the animation properties.

We do this first so we can land this change separately to ease bisecting any
regressions it might trigger.

--HG--
extra : commitid : KzukSO91RMH
extra : rebase_source : 54ed2aeb69d8bceca424c70c7f33d4bf92d54546

dom/animation/Animation.cpp

@@ -355,8 +355,13 @@ Animation::TriggerOnNextTick(const Nullable<TimeDuration>& aReadyTime)
 void
 Animation::TriggerNow()
 {
-  MOZ_ASSERT(PlayState() == AnimationPlayState::Pending,
-             "Expected to start a pending animation");
+  // Normally we expect the play state to be pending but when an animation
+  // is cancelled and its rendered document can't be reached, we can end up
+  // with the animation still in a pending player tracker even after it is
+  // no longer pending.
+  if (PlayState() != AnimationPlayState::Pending) {
+    return;
+  }
   MOZ_ASSERT(mTimeline && !mTimeline->GetCurrentTime().IsNull(),
              "Expected an active timeline");
 

layout/style/AnimationCommon.cpp

@@ -13,6 +13,7 @@
 #include "nsCSSPropertySet.h"
 #include "nsCSSValue.h"
 #include "nsCycleCollectionParticipant.h"
+#include "nsDOMMutationObserver.h"
 #include "nsStyleContext.h"
 #include "nsIFrame.h"
 #include "nsLayoutUtils.h"
@@ -770,6 +771,13 @@ AnimationCollection::PropertyDtor(void *aObject, nsIAtom *aPropertyName,
   MOZ_ASSERT(!collection->mCalledPropertyDtor, "can't call dtor twice");
   collection->mCalledPropertyDtor = true;
 #endif
+  {
+    nsAutoAnimationMutationBatch mb(collection->mElement);
+
+    for (size_t animIdx = collection->mAnimations.Length(); animIdx-- != 0; ) {
+      collection->mAnimations[animIdx]->CancelFromStyle();
+    }
+  }
   delete collection;
 }
 

layout/style/AnimationCommon.h

@@ -272,9 +272,6 @@ struct AnimationCollection : public PRCList
 
   void Destroy()
   {
-    for (size_t animIdx = mAnimations.Length(); animIdx-- != 0; ) {
-      mAnimations[animIdx]->CancelFromStyle();
-    }
     // This will call our destructor.
     mElement->DeleteProperty(mElementProperty);
   }