Introduce DisplayCompositor Mojo API for Mus

components/mus/surfaces/compositor_frame_sink_delegate.h

Index: components/mus/surfaces/compositor_frame_sink_delegate.h
diff --git a/components/mus/surfaces/compositor_frame_sink_delegate.h b/components/mus/surfaces/compositor_frame_sink_delegate.h
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+++ b/components/mus/surfaces/compositor_frame_sink_delegate.h
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+// Copyright 2016 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 COMPONENTS_MUS_SURFACES_COMPOSITOR_FRAME_SINK_DELEGATE_H_
+#define COMPONENTS_MUS_SURFACES_COMPOSITOR_FRAME_SINK_DELEGATE_H_
+
+#include "cc/surfaces/surface_id.h"
+
+namespace mus {
+namespace gpu {
+
+class CompositorFrameSinkImpl;
+
+// A CompositorFrameSinkDelegate decouples CompositorFrameSinks from their
+// factories enabling them to be unit tested.
+class CompositorFrameSinkDelegate {
+ public:
+  virtual void CompositorFrameSinkConnectionLost(int sink_id) = 0;
+
+  virtual cc::SurfaceId GenerateSurfaceId() = 0;
+};
+
+}  // namespace gpu
+}  // namespace mus
+
+#endif  // COMPONENTS_MUS_SURFACES_COMPOSITOR_FRAME_SINK_DELEGATE_H_