Bug 1123196 part 2 - Don't resolve pending animations for nested calls to PaintRoot; r=mattwoodrow

We resolve pending animations when painting has finished in
nsDisplayList::PaintRoot. However, in the process we can trigger nested calls to
PaintFrame (e.g. due to use of -moz-element). In that case, we shouldn't resolve
pending animations until we complete the widget transaction.

This patch adds a check that we're in a widget transaction before we resolve
pending animations.

layout/base/nsDisplayList.cpp

@@ -1688,7 +1688,7 @@ already_AddRefed<LayerManager> nsDisplayList::PaintRoot(nsDisplayListBuilder* aB
   aBuilder->SetIsCompositingCheap(temp);
   layerBuilder->DidEndTransaction();
 
-  if (document) {
+  if (document && widgetTransaction) {
     StartPendingAnimations(document, layerManager->GetAnimationReadyTime());
   }