cc: Make OutputSurface::SwapBuffers pure virtual

mojo/cc/direct_output_surface.h

+// Copyright 2014 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 MOJO_CC_DIRECT_OUTPUT_SURFACE_H_
+#define MOJO_CC_DIRECT_OUTPUT_SURFACE_H_
+
+#include "cc/output/output_surface.h"
+
+namespace mojo {
+
+class DirectOutputSurface : public cc::OutputSurface {
+ public:
+  explicit DirectOutputSurface(
+      const scoped_refptr<cc::ContextProvider>& context_provider);
+  virtual ~DirectOutputSurface() override;
+
+  // cc::OutputSurface implementation
+  virtual void SwapBuffers(cc::CompositorFrame* frame) override;
+
+private:
+  base::WeakPtrFactory<DirectOutputSurface> weak_ptr_factory_;
+};
+
+}  // namespace mojo
+
+#endif  // MOJO_CC_DIRECT_OUTPUT_SURFACE_H_