Revert of 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 56d244c41301e10ff3c18394a554b065c8519501..dfce8088d18ac248bfef847f62a4ac35231bc3ac 100644
--- a/cc/trees/single_thread_proxy.cc
+++ b/cc/trees/single_thread_proxy.cc
@@ -90,7 +90,6 @@
   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,
@@ -427,10 +426,13 @@
     // 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();
+    // Since activation could cause tasks to run, post CommitComplete
+    // separately so that it runs after these tasks.  This is the loose
+    // equivalent of blocking commit until activation and also running
+    // all tasks posted during commit/activation before CommitComplete.
+    MainThreadTaskRunner()->PostTask(
+        FROM_HERE, base::Bind(&SingleThreadProxy::CommitComplete,
+                              weak_factory_.GetWeakPtr()));
   }
 
   timing_history_.DidActivateSyncTree();