Bug 1302648 part 3 - Change order of releasing owning element when cancel animation. r=birtles

The first step of Animation::Cancel(), it will release owning element.
However we will use owning element for queueing the CSS-Animations event, So we will need to release owning element after calling Animation::Cancel() in order to fire the animationcancel event.

MozReview-Commit-ID: ATqkIGkqREx

--HG--
extra : rebase_source : 9b04dc59349d1203d5f8604516fcc6765cf3d5a7

layout/style/nsAnimationManager.h

@@ -110,7 +110,6 @@ public:
   void PauseFromStyle();
   void CancelFromStyle() override
   {
-    mOwningElement = OwningElementRef();
 
     // When an animation is disassociated with style it enters an odd state
     // where its composite order is undefined until it first transitions
@@ -126,6 +125,11 @@ public:
     mNeedsNewAnimationIndexWhenRun = true;
 
     Animation::CancelFromStyle();
+
+    // We need to do this *after* calling CancelFromStyle() since
+    // CancelFromStyle might synchronously trigger a cancel event for which
+    // we need an owning element to target the event at.
+    mOwningElement = OwningElementRef();
   }
 
   void Tick() override;