cc: Unregister FrameSink hierarchy on CompositorFrameSinkSupport destruction

cc/surfaces/compositor_frame_sink_support.h

Index: cc/surfaces/compositor_frame_sink_support.h
diff --git a/cc/surfaces/compositor_frame_sink_support.h b/cc/surfaces/compositor_frame_sink_support.h
index 1d89554e025567337fc70b2f21464bccc4c1b434..239e19425de25b514d68301b707a1e33837b85f0 100644
--- a/cc/surfaces/compositor_frame_sink_support.h
+++ b/cc/surfaces/compositor_frame_sink_support.h
@@ -94,6 +94,9 @@ class CC_SURFACES_EXPORT CompositorFrameSinkSupport
   // Whether or not a frame observer has been added.
   bool added_frame_observer_ = false;
 
+  // The set of BeginFrame children of this CompositorFrameSink.
+  std::unordered_set<FrameSinkId, FrameSinkIdHash> child_frame_sinks_;
+
   base::WeakPtrFactory<CompositorFrameSinkSupport> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(CompositorFrameSinkSupport);