Make ui::Compositor use ui::Scheduler

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 488bba48a445c84c966349def4a8340cfa25ac8d..c8eb6b0fad6b33dc9811cb2169a976d8b519ab8e 100644
--- a/cc/trees/single_thread_proxy.cc
+++ b/cc/trees/single_thread_proxy.cc
@@ -91,6 +91,7 @@ void SingleThreadProxy::SetLayerTreeHostClientReady() {
   if (layer_tree_host_->settings().single_thread_proxy_scheduler &&
       !scheduler_on_impl_thread_) {
     SchedulerSettings scheduler_settings(layer_tree_host_->settings());
+    // SingleThreadProxy should run in main thread low latency mode.
     scheduler_settings.main_thread_should_always_be_low_latency = true;
     scheduler_on_impl_thread_ =
         Scheduler::Create(this,
@@ -437,6 +438,9 @@ void SingleThreadProxy::DidActivateSyncTree() {
     // the pending tree is not actually ready in the SingleThreadProxy.
     layer_tree_host_impl_->SetRequiresHighResToDraw();
 
+    // Synchronously call to CommitComplete. Resetting
+    // |commit_blocking_task_runner| would make sure all tasks posted during
+    // commit/activation before CommitComplete.
     CommitComplete();
   }