Refactor external begin frame source into a base class

cc/scheduler/begin_frame_source.h

 // Copyright 2014 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_SCHEDULER_BEGIN_FRAME_SOURCE_H_
 #define CC_SCHEDULER_BEGIN_FRAME_SOURCE_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 183 matching lines @@
                                       BeginFrameArgs::BeginFrameArgsType type);
 
   std::unique_ptr<DelayBasedTimeSource> time_source_;
   std::unordered_set<BeginFrameObserver*> observers_;
   base::TimeTicks last_timebase_;
   base::TimeDelta authoritative_interval_;
 
   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_