cc: Unregister FrameSink hierarchy on CompositorFrameSinkSupport destruction

cc/surfaces/compositor_frame_sink_support.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_SURFACES_COMPOSITOR_FRAME_SINK_SUPPORT_H_
 #define CC_SURFACES_COMPOSITOR_FRAME_SINK_SUPPORT_H_
 
 #include "base/compiler_specific.h"
 #include "base/memory/weak_ptr.h"
 #include "cc/output/compositor_frame.h"
@@ skipping 76 matching lines @@
 
   // The last begin frame args generated by the begin frame source.
   BeginFrameArgs last_begin_frame_args_;
 
   // Whether a request for begin frames has been issued.
   bool needs_begin_frame_ = false;
 
   // 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);
 };
 
 }  // namespace cc
 
 #endif  // CC_SURFACES_COMPOSITOR_FRAME_SINK_SUPPORT_H_