Add observers to FrameSinkManagerHost.

content/browser/compositor/frame_sink_observer.h

+// Copyright 2017 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 CONTENT_BROWSER_COMPOSITOR_FRAME_SINK_OBSERVER_H_
+#define CONTENT_BROWSER_COMPOSITOR_FRAME_SINK_OBSERVER_H_
+
+namespace cc {
+class SurfaceInfo;
+}
+
+namespace content {
+
+class FrameSinkObserver {
+ public:
+  // Runs when a CompositorFrame is received for the given SurfaceInfo for the
+  // first time.
+  virtual void OnSurfaceCreated(const cc::SurfaceInfo& surface_info) = 0;
+
+ protected:
+  ~FrameSinkObserver() {}
+};
+
+}  // namespace content
+
+#endif  // CONTENT_BROWSER_COMPOSITOR_FRAME_SINK_OBSERVER_H_