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

cc/surfaces/primary_begin_frame_source.h

Index: cc/surfaces/primary_begin_frame_source.h
diff --git a/cc/surfaces/primary_begin_frame_source.h b/cc/surfaces/primary_begin_frame_source.h
new file mode 100644
index 0000000000000000000000000000000000000000..e881a29727127e876e572e262fccb2f1c74e4af2
--- /dev/null
+++ b/cc/surfaces/primary_begin_frame_source.h
@@ -0,0 +1,56 @@
+// 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_PRIMARY_BEGIN_FRAME_SOURCE_H_
+#define CC_SURFACES_PRIMARY_BEGIN_FRAME_SOURCE_H_
+
+#include "cc/scheduler/begin_frame_source.h"
+
+namespace cc {
+
+// PrimaryBeginFrameSource echos the first BeginFrameSource in the system.
+// If the first source goes away then it will echo the new first
+// BeginFrameSource.
+class PrimaryBeginFrameSource : public BeginFrameSource,
+                                public BeginFrameObserver,
+                                public ExternalBeginFrameSourceClient {
+ public:
+  PrimaryBeginFrameSource();
+  ~PrimaryBeginFrameSource() override;
+
+  void OnBeginFrameSourceAdded(BeginFrameSource* begin_frame_source);
+  void OnBeginFrameSourceRemoved(BeginFrameSource* begin_frame_source);
+
+  // BeginFrameObserver implementation.
+  void OnBeginFrame(const BeginFrameArgs& args) override;
+  const BeginFrameArgs& LastUsedBeginFrameArgs() const override;
+  void OnBeginFrameSourcePausedChanged(bool paused) override;
+
+  // BeginFrameSource implementation.
+  void DidFinishFrame(BeginFrameObserver* obs,
+                      const BeginFrameAck& ack) override;
+  void AddObserver(BeginFrameObserver* obs) override;
+  void RemoveObserver(BeginFrameObserver* obs) override;
+  bool IsThrottled() const override;
+
+  // ExternalBeginFrameSourceClient implementation.
+  void OnNeedsBeginFrames(bool needs_begin_frames) override;
+
+ private:
+  ExternalBeginFrameSource begin_frame_source_;
+  BeginFrameSource* current_begin_frame_source_ = nullptr;
+
+  // The last begin frame args generated by the begin frame source.
+  BeginFrameArgs last_begin_frame_args_;
+
+  bool needs_begin_frames_ = false;
+
+  base::flat_set<BeginFrameSource*> sources_;
+
+  DISALLOW_COPY_AND_ASSIGN(PrimaryBeginFrameSource);
+};
+
+}  // namespace cc
+
+#endif  // CC_SURFACES_PRIMARY_BEGIN_FRAME_SOURCE_H_