cc: Reject CompositorFrames to old child surfaces

cc/output/compositor_frame_metadata.h

Index: cc/output/compositor_frame_metadata.h
diff --git a/cc/output/compositor_frame_metadata.h b/cc/output/compositor_frame_metadata.h
index 0b34d316400b63735e10b51f37a25f9c9c7a8dac..ac4c79a42a221801e831f0b1bc2189c426c08c0b 100644
--- a/cc/output/compositor_frame_metadata.h
+++ b/cc/output/compositor_frame_metadata.h
@@ -90,6 +90,12 @@ class CC_EXPORT CompositorFrameMetadata {
   // that if |can_activate_before_dependencies| then the display compositor
   // can choose to activate a CompositorFrame before all dependencies are
   // available.
+  // Note: |activation_dependencies| and |referenced_surfaces| are disjointed
+  //       sets of surface IDs. If a surface ID is known to exist and can be
+  //       used without additional synchronization, then it is placed in
+  //       |referenced_surfaces|. |activation_dependencies| is the set of
+  //       surface IDs that this frame would like to block on until they
+  //       become available or a deadline hits.
   std::vector<SurfaceId> activation_dependencies;
 
   // This indicates whether this CompositorFrame can be activated before