Refactor external begin frame source into a base class

cc/scheduler/begin_frame_source.h

Index: cc/scheduler/begin_frame_source.h
diff --git a/cc/scheduler/begin_frame_source.h b/cc/scheduler/begin_frame_source.h
index 0a8a51f08d7c911d6c1594b1623ae74ad5a9fe43..4a0668cd9fb4534deea2cb214e39c917ea70ee12 100644
--- a/cc/scheduler/begin_frame_source.h
+++ b/cc/scheduler/begin_frame_source.h
@@ -201,6 +201,41 @@ class CC_EXPORT DelayBasedBeginFrameSource : public SyntheticBeginFrameSource,
   DISALLOW_COPY_AND_ASSIGN(DelayBasedBeginFrameSource);
 };
 
+class CC_EXPORT ExternalBeginFrameSourceClient {
+ public:
+  // Only called when changed.  Assumed false by default.
+  virtual void OnNeedsBeginFrames(bool needs_begin_frames) = 0;
+};
+
+// A BeginFrameSource that is only ticked manually.  Usually the endpoint
+// of messages from some other thread/process that send OnBeginFrame and
+// receive SetNeedsBeginFrame messages.  This turns such messages back into
+// an observable BeginFrameSource.
+class CC_EXPORT ExternalBeginFrameSource : public BeginFrameSource {
+ public:
+  // Client lifetime must be preserved by owner past the lifetime of this class.
+  explicit ExternalBeginFrameSource(ExternalBeginFrameSourceClient* client);
+  ~ExternalBeginFrameSource() override;
+
+  // BeginFrameSource implementation.
+  void AddObserver(BeginFrameObserver* obs) override;
+  void RemoveObserver(BeginFrameObserver* obs) override;
+  void DidFinishFrame(BeginFrameObserver* obs,
+                      size_t remaining_frames) override {}
+
+  void OnSetBeginFrameSourcePaused(bool paused);
+  void OnBeginFrame(const BeginFrameArgs& args);
+
+ protected:
+  BeginFrameArgs missed_begin_frame_args_;
+  std::unordered_set<BeginFrameObserver*> observers_;
+  ExternalBeginFrameSourceClient* client_;
+  bool paused_ = false;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(ExternalBeginFrameSource);
+};
+
 }  // namespace cc
 
 #endif  // CC_SCHEDULER_BEGIN_FRAME_SOURCE_H_