(Reland) Discard compositor frames from unloaded web content

cc/ipc/compositor_frame_metadata_struct_traits.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_IPC_COMPOSITOR_FRAME_METADATA_STRUCT_TRAITS_H_
 #define CC_IPC_COMPOSITOR_FRAME_METADATA_STRUCT_TRAITS_H_
 
 #include "cc/ipc/compositor_frame_metadata.mojom-shared.h"
 #include "cc/output/compositor_frame_metadata.h"
 
@@ skipping 93 matching lines @@
   static const std::vector<cc::SurfaceId>& referenced_surfaces(
       const cc::CompositorFrameMetadata& metadata) {
     return metadata.referenced_surfaces;
   }
 
   static bool can_activate_before_dependencies(
       const cc::CompositorFrameMetadata& metadata) {
     return metadata.can_activate_before_dependencies;
   }
 
+  static uint32_t content_source_id(
+      const cc::CompositorFrameMetadata& metadata) {
+    return metadata.content_source_id;
+  }
+
   static bool Read(cc::mojom::CompositorFrameMetadataDataView data,
                    cc::CompositorFrameMetadata* out);
 };
 
 }  // namespace mojo
 
 #endif  // CC_IPC_COMPOSITOR_FRAME_METADATA_STRUCT_TRAITS_H_