Make disable vsync run the renderer independently

cc/test/fake_external_begin_frame_source.cc

Index: cc/test/fake_external_begin_frame_source.cc
diff --git a/cc/test/fake_external_begin_frame_source.cc b/cc/test/fake_external_begin_frame_source.cc
index 9beff1540566e6df3001846178205339ca1ee42d..1a15b09b49ab69a2aa0a9233efe04493147cff3b 100644
--- a/cc/test/fake_external_begin_frame_source.cc
+++ b/cc/test/fake_external_begin_frame_source.cc
@@ -58,6 +58,10 @@ void FakeExternalBeginFrameSource::RemoveObserver(BeginFrameObserver* obs) {
     client_->OnRemoveObserver(obs);
 }
 
+bool FakeExternalBeginFrameSource::IsThrottled() const {
+  return true;
+}
+
 void FakeExternalBeginFrameSource::TestOnBeginFrame(
     const BeginFrameArgs& args) {
   DCHECK(CalledOnValidThread());