Move default begin frame sources out to mus

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 cf9206c939011ef6b4965eb93e2b2745ee3abb70..4237f4d1de383b0ad48636717dd9b2b0d53b875c 100644
--- a/cc/trees/single_thread_proxy.cc
+++ b/cc/trees/single_thread_proxy.cc
@@ -67,35 +67,17 @@ void SingleThreadProxy::Start(
     SchedulerSettings scheduler_settings(
         layer_tree_host_->settings().ToSchedulerSettings());
     scheduler_settings.commit_to_active_tree = CommitToActiveTree();
-
     std::unique_ptr<CompositorTimingHistory> compositor_timing_history(
         new CompositorTimingHistory(
             scheduler_settings.using_synchronous_renderer_compositor,
             CompositorTimingHistory::BROWSER_UMA,
             layer_tree_host_->rendering_stats_instrumentation()));
 
-    BeginFrameSource* frame_source = nullptr;
-    if (!layer_tree_host_->settings().use_output_surface_begin_frame_source) {
-      frame_source = external_begin_frame_source_.get();
-      if (!scheduler_settings.throttle_frame_production) {
-        // Unthrottled source takes precedence over external sources.
-        unthrottled_begin_frame_source_.reset(new BackToBackBeginFrameSource(
-            base::MakeUnique<DelayBasedTimeSource>(
-                task_runner_provider_->MainThreadTaskRunner())));
-        frame_source = unthrottled_begin_frame_source_.get();
-      }
-      if (!frame_source) {
-        synthetic_begin_frame_source_.reset(new DelayBasedBeginFrameSource(
-            base::MakeUnique<DelayBasedTimeSource>(
-                task_runner_provider_->MainThreadTaskRunner())));
-        frame_source = synthetic_begin_frame_source_.get();
-      }
-    }
-
+    DCHECK(layer_tree_host_->settings().use_output_surface_begin_frame_source);
     scheduler_on_impl_thread_ =
         Scheduler::Create(this, scheduler_settings, layer_tree_host_->id(),
                           task_runner_provider_->MainThreadTaskRunner(),
-                          frame_source, std::move(compositor_timing_history));
+                          nullptr, std::move(compositor_timing_history));
   }
 
   layer_tree_host_impl_ = layer_tree_host_->CreateLayerTreeHostImpl(this);
@@ -483,11 +465,8 @@ void SingleThreadProxy::CommitVSyncParameters(base::TimeTicks timebase,
 }
 
 void SingleThreadProxy::SetBeginFrameSource(BeginFrameSource* source) {
+  DCHECK(layer_tree_host_->settings().use_output_surface_begin_frame_source);
   DCHECK(layer_tree_host_->settings().single_thread_proxy_scheduler);
-  // TODO(enne): this overrides any preexisting begin frame source.  Those
-  // other sources will eventually be removed and this will be the only path.
-  if (!layer_tree_host_->settings().use_output_surface_begin_frame_source)
-    return;
   if (scheduler_on_impl_thread_)
     scheduler_on_impl_thread_->SetBeginFrameSource(source);
 }