cc: Add OutputSurface state snapshots.

cc/test/fake_output_surface_client.h

Index: cc/test/fake_output_surface_client.h
diff --git a/cc/test/fake_output_surface_client.h b/cc/test/fake_output_surface_client.h
index c511c2655d294fb4a94dc3109d3f7dd7e9695c14..6b31e17ae16509857673879f7477c46bb7ddf2f6 100644
--- a/cc/test/fake_output_surface_client.h
+++ b/cc/test/fake_output_surface_client.h
@@ -19,6 +19,9 @@ class FakeOutputSurfaceClient : public OutputSurfaceClient {
   void DidReceiveSwapBuffersAck() override;
   void DidReceiveTextureInUseResponses(
       const gpu::TextureInUseResponses& responses) override {}
+  void ReadbackSwappedOverlayTextures(
+      std::vector<SkBitmap>* bitmaps,
+      const base::Callback<void(bool)>& callback) override {}
 
   int swap_count() { return swap_count_; }