cc:: Change plumbing for external_begin_frame_source to the Scheduler.

cc/test/fake_proxy.h

Index: cc/test/fake_proxy.h
diff --git a/cc/test/fake_proxy.h b/cc/test/fake_proxy.h
index 7998625ef77082a26b5345c409947d46d5bbbec0..341d54142eb8c91b6fa25024cfe6bd32a49178b1 100644
--- a/cc/test/fake_proxy.h
+++ b/cc/test/fake_proxy.h
@@ -35,7 +35,8 @@ class FakeProxy : public Proxy {
   void MainThreadHasStoppedFlinging() override {}
   bool BeginMainFrameRequested() const override;
   bool CommitRequested() const override;
-  void Start() override {}
+  void Start(
+      scoped_ptr<BeginFrameSource> external_begin_frame_source) override {}
   void Stop() override {}
   bool SupportsImplScrolling() const override;
   bool MainFrameWillHappenForTesting() override;