(Reland) Discard compositor frames from unloaded web content

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 64 matching lines @@
 
   std::vector<ui::LatencyInfo> latency_info;
 
   // This is the set of Surfaces that are referenced by this frame.
   std::vector<SurfaceId> referenced_surfaces;
 
   // 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;
+
  private:
   CompositorFrameMetadata(const CompositorFrameMetadata& other);
   CompositorFrameMetadata operator=(const CompositorFrameMetadata&) = delete;
 };
 
 }  // namespace cc
 
 #endif  // CC_OUTPUT_COMPOSITOR_FRAME_METADATA_H_