cc: Reject CompositorFrames to old child surfaces

cc/output/compositor_frame_metadata.h

 // Copyright (c) 2012 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_OUTPUT_COMPOSITOR_FRAME_METADATA_H_
 #define CC_OUTPUT_COMPOSITOR_FRAME_METADATA_H_
 
 #include <stdint.h>
 
 #include <vector>
@@ skipping 72 matching lines @@
   // determine which surfaces to retain and which to evict. It will likely
   // be unnecessary for the embedder to explicitly specify which surfaces to
   // retain. Thus, this field will likely go away.
   std::vector<SurfaceId> referenced_surfaces;
 
   // This is the set of dependent SurfaceIds that should be active in the
   // display compositor before this CompositorFrame can be activated. Note
   // 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
   // dependencies have been resolved.
   bool can_activate_before_dependencies = true;
 
   // This is a value that allows the browser to associate compositor frames
   // with the content that they represent -- typically top-level page loads.
   // TODO(kenrb, fsamuel): This should eventually by SurfaceID, when they
   // become available in all renderer processes. See https://crbug.com/695579.
   uint32_t content_source_id = 0;
 
   // BeginFrameAck for the BeginFrame that this CompositorFrame answers.
   BeginFrameAck begin_frame_ack;
 
   // Once the display compositor processes a frame containing a non-zero frame
   // token, the token is sent to embedder of the frame. This is helpful when
   // the embedder wants to do something after a particular frame is processed.
   uint32_t frame_token = 0;
 
  private:
   CompositorFrameMetadata(const CompositorFrameMetadata& other);
   CompositorFrameMetadata operator=(const CompositorFrameMetadata&) = delete;
 };
 
 }  // namespace cc
 
 #endif  // CC_OUTPUT_COMPOSITOR_FRAME_METADATA_H_