Decouple FrameSink Hierarchy registration from SurfaceFactoryClient registration

cc/surfaces/surface_manager.h

Index: cc/surfaces/surface_manager.h
diff --git a/cc/surfaces/surface_manager.h b/cc/surfaces/surface_manager.h
index 569c6e44c0306d3ae694b4adb5a1dd1a918ccd7f..e4b6f17c5f6efd6e22edeca7795c5dfe9db6748d 100644
--- a/cc/surfaces/surface_manager.h
+++ b/cc/surfaces/surface_manager.h
@@ -251,14 +251,16 @@ class CC_SURFACES_EXPORT SurfaceManager {
     FrameSinkSourceMapping();
     FrameSinkSourceMapping(const FrameSinkSourceMapping& other);
     ~FrameSinkSourceMapping();
-    bool is_empty() const { return !client && children.empty(); }
-    // The client that's responsible for creating this namespace.  Never null.
-    SurfaceFactoryClient* client;
+    bool has_children() const { return !children.empty(); }
     // The currently assigned begin frame source for this client.
     BeginFrameSource* source;
     // This represents a dag of parent -> children mapping.
     std::vector<FrameSinkId> children;
   };
+
+  std::unordered_map<FrameSinkId, SurfaceFactoryClient*, FrameSinkIdHash>
+      clients_;
+
   std::unordered_map<FrameSinkId, FrameSinkSourceMapping, FrameSinkIdHash>
       frame_sink_source_map_;