MojoCompositorFrameSinkPrivate should support copy requests

cc/output/copy_output_request.h

Index: cc/output/copy_output_request.h
diff --git a/cc/output/copy_output_request.h b/cc/output/copy_output_request.h
index 112daa40dcefe08ad0b4c567badf096b378e2778..1b3898359be798ca0f7d36741e787c813fcb2871 100644
--- a/cc/output/copy_output_request.h
+++ b/cc/output/copy_output_request.h
@@ -83,6 +83,14 @@ class CC_EXPORT CopyOutputRequest {
 
   void SendResult(std::unique_ptr<CopyOutputResult> result);
 
+  void set_result_callback(CopyOutputRequestCallback result_callback) {
+    result_callback_ = result_callback;
+  }
+
+  const CopyOutputRequestCallback& result_callback() {
+    return result_callback_;
+  }
+
  private:
   friend struct mojo::StructTraits<mojom::CopyOutputRequestDataView,
                                    std::unique_ptr<CopyOutputRequest>>;