cc: Introduce BeginFrame sequence numbers and acknowledgements.

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 8f2d4f0998e862ba9ddd489c91d77adfbce6f232..fea5456e7616798eda5119b20f19da7a49bc154d 100644
--- a/cc/test/fake_external_begin_frame_source.h
+++ b/cc/test/fake_external_begin_frame_source.h
@@ -10,6 +10,7 @@
 #include "base/cancelable_callback.h"
 #include "base/memory/weak_ptr.h"
 #include "base/threading/non_thread_safe.h"
+#include "cc/output/begin_frame_args.h"
 #include "cc/scheduler/begin_frame_source.h"
 
 namespace cc {
@@ -35,7 +36,7 @@ class FakeExternalBeginFrameSource
   void AddObserver(BeginFrameObserver* obs) override;
   void RemoveObserver(BeginFrameObserver* obs) override;
   void DidFinishFrame(BeginFrameObserver* obs,
-                      size_t remaining_frames) override {}
+                      const BeginFrameAck& ack) override {}
   bool IsThrottled() const override;
 
   void TestOnBeginFrame(const BeginFrameArgs& args);
@@ -49,6 +50,7 @@ class FakeExternalBeginFrameSource
   const double milliseconds_per_frame_;
   Client* client_ = nullptr;
   bool paused_ = false;
+  uint64_t next_begin_frame_number_ = BeginFrameArgs::kStartingFrameNumber;
   std::set<BeginFrameObserver*> observers_;
   base::CancelableCallback<void(const BeginFrameArgs&)> begin_frame_task_;
   base::WeakPtrFactory<FakeExternalBeginFrameSource> weak_ptr_factory_;