Revert of 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 59023b4341b890e6d695dff54424be5e3a1d30d6..e383e5069dcd82c0f9c4a044fd4f38d6eabc4969 100644
--- a/cc/output/compositor_frame_metadata.h
+++ b/cc/output/compositor_frame_metadata.h
@@ -82,12 +82,6 @@
   // 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;
-
  private:
   CompositorFrameMetadata(const CompositorFrameMetadata& other);
   CompositorFrameMetadata operator=(const CompositorFrameMetadata&) = delete;