Discard compositor frames from unloaded web content

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 e383e5069dcd82c0f9c4a044fd4f38d6eabc4969..8bd04f00d18ce426d4ff098a969ba8556f520f8a 100644
--- a/cc/output/compositor_frame_metadata.h
+++ b/cc/output/compositor_frame_metadata.h
@@ -82,6 +82,12 @@ class CC_EXPORT CompositorFrameMetadata {
   // 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;