Bug 1843847. Clear nsDisplayListBuilder::mCurrentContainerASR at the end of a paint. r=mstange, a=dsmith

Differential Revision: https://phabricator.services.mozilla.com/D183741

layout/painting/nsDisplayList.cpp

@@ -790,6 +790,7 @@ void nsDisplayListBuilder::AddEffectUpdate(dom::RemoteBrowser* aBrowser,
 void nsDisplayListBuilder::EndFrame() {
   NS_ASSERTION(!mInInvalidSubtree,
                "Someone forgot to cleanup mInInvalidSubtree!");
+  mCurrentContainerASR = nullptr;
   mActiveScrolledRoots.Clear();
   mEffectsUpdates.Clear();
   FreeClipChains();