cc::SurfaceDependencyTracker should not crash when a Display goes away

cc/surfaces/frame_sink_observer.h

Index: cc/surfaces/frame_sink_observer.h
diff --git a/cc/surfaces/frame_sink_observer.h b/cc/surfaces/frame_sink_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..f3cccd7f4928da955bd31916011bfb8d3bf2974a
--- /dev/null
+++ b/cc/surfaces/frame_sink_observer.h
@@ -0,0 +1,23 @@
+// 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 CC_SURFACES_FRAME_SINK_OBSERVER_H_
+#define CC_SURFACES_FRAME_SINK_OBSERVER_H_
+
+namespace cc {
+
+class BeginFrameSource;
+
+class FrameSinkObserver {
+ public:
+  virtual void OnBeginFrameSourceAdded(
+      BeginFrameSource* begin_frame_source) = 0;
+
+  virtual void OnBeginFrameSourceRemoved(
+      BeginFrameSource* begin_frame_source) = 0;
+};
+
+}  // namespace cc
+
+#endif  // CC_SURFACES_FRAME_SINK_OBSERVER_H_