[wip] hack to use synthetic bfs for browser compositor

cc/trees/single_thread_proxy.cc

Index: cc/trees/single_thread_proxy.cc
diff --git a/cc/trees/single_thread_proxy.cc b/cc/trees/single_thread_proxy.cc
index f30413a15aeae9f665325a75e8101f5efcceb1ad..10fd224f89dc461c181d319796ff43b6f9eaafd0 100644
--- a/cc/trees/single_thread_proxy.cc
+++ b/cc/trees/single_thread_proxy.cc
@@ -416,8 +416,15 @@ void SingleThreadProxy::DidLoseCompositorFrameSinkOnImplThread() {
 }
 
 void SingleThreadProxy::SetBeginFrameSource(BeginFrameSource* source) {
-  if (scheduler_on_impl_thread_)
-    scheduler_on_impl_thread_->SetBeginFrameSource(source);
+  if (!synthetic_begin_frame_source_) {
+    synthetic_begin_frame_source_.reset(
+        new DelayBasedBeginFrameSource(base::MakeUnique<DelayBasedTimeSource>(
+            task_runner_provider_->MainThreadTaskRunner())));
+  }
+  if (scheduler_on_impl_thread_) {
+    scheduler_on_impl_thread_->SetBeginFrameSource(
+        synthetic_begin_frame_source_.get());
+  }
 }
 
 void SingleThreadProxy::DidReceiveCompositorFrameAckOnImplThread() {