cc: Make BackToBackBeginFrameSource a SyntheticBeginFrameSource.

cc/test/fake_external_begin_frame_source.h

Index: cc/test/fake_external_begin_frame_source.h
diff --git a/cc/test/fake_external_begin_frame_source.h b/cc/test/fake_external_begin_frame_source.h
index 1653decf8cc86465af54b7859f06d9a28e74bff7..5839024d3a358681853640bd087eda609a9522db 100644
--- a/cc/test/fake_external_begin_frame_source.h
+++ b/cc/test/fake_external_begin_frame_source.h
@@ -5,27 +5,48 @@
 #ifndef CC_TEST_FAKE_EXTERNAL_BEGIN_FRAME_SOURCE_H_
 #define CC_TEST_FAKE_EXTERNAL_BEGIN_FRAME_SOURCE_H_
 
+#include <set>
+
+#include "base/cancelable_callback.h"
 #include "base/memory/weak_ptr.h"
 #include "cc/scheduler/begin_frame_source.h"
 
 namespace cc {
 
 class FakeExternalBeginFrameSource
-    : public BeginFrameSourceBase,
+    : public BeginFrameSource,
       public NON_EXPORTED_BASE(base::NonThreadSafe) {
  public:
-  explicit FakeExternalBeginFrameSource(double refresh_rate);
+  class Client {
+   public:
+    virtual void OnAddObserver(BeginFrameObserver* obs) = 0;
+    virtual void OnRemoveObserver(BeginFrameObserver* obs) = 0;
+  };
+
+  explicit FakeExternalBeginFrameSource(double refresh_rate,
+                                        bool tick_automatically);
   ~FakeExternalBeginFrameSource() override;
 
-  // BeginFrameSourceBase overrides.
-  void OnNeedsBeginFramesChanged(bool needs_begin_frames) override;
+  void SetClient(Client* client) { client_ = client; }
+  void SetPaused(bool paused);
 
-  void TestOnBeginFrame();
-  void PostTestOnBeginFrame();
+  // BeginFrameSource implementation.
+  void AddObserver(BeginFrameObserver* obs) override;
+  void RemoveObserver(BeginFrameObserver* obs) override;
+  void DidFinishFrame(BeginFrameObserver* obs,
+                      size_t remaining_frames) override {}
+
+  void TestOnBeginFrame(const BeginFrameArgs& args);
 
  private:
-  double milliseconds_per_frame_;
-  base::CancelableClosure begin_frame_task_;
+  void PostTestOnBeginFrame();
+
+  const bool tick_automatically_;
+  const double milliseconds_per_frame_;
+  Client* client_ = nullptr;
+  bool paused_ = false;
+  std::set<BeginFrameObserver*> observers_;
+  base::CancelableCallback<void(const BeginFrameArgs&)> begin_frame_task_;
   base::WeakPtrFactory<FakeExternalBeginFrameSource> weak_ptr_factory_;
 };