compositor worker: Use a WebThread for the compositor thread.

components/scheduler/child/webthread_impl_for_worker_scheduler.cc

Index: components/scheduler/child/webthread_impl_for_worker_scheduler.cc
diff --git a/components/scheduler/child/webthread_impl_for_worker_scheduler.cc b/components/scheduler/child/webthread_impl_for_worker_scheduler.cc
index afdcb6d50322a49b1a1a1f83c594086ac5b3f467..13d23d7fe8ea8dd893d4b3ddcc1d630d44cc0f03 100644
--- a/components/scheduler/child/webthread_impl_for_worker_scheduler.cc
+++ b/components/scheduler/child/webthread_impl_for_worker_scheduler.cc
@@ -29,7 +29,9 @@ WebThreadImplForWorkerScheduler::WebThreadImplForWorkerScheduler(
   bool started = thread_->StartWithOptions(options);
   CHECK(started);
   thread_task_runner_ = thread_->task_runner();
+}
 
+void WebThreadImplForWorkerScheduler::Init() {
   base::WaitableEvent completion(false, false);
   thread_task_runner_->PostTask(
       FROM_HERE, base::Bind(&WebThreadImplForWorkerScheduler::InitOnThread,
@@ -56,7 +58,10 @@ void WebThreadImplForWorkerScheduler::InitOnThread(
       thread_->message_loop(), make_scoped_ptr(new base::DefaultTickClock()));
   worker_scheduler_ = WorkerScheduler::Create(task_runner_delegate_);
   worker_scheduler_->Init();
-  task_runner_ = worker_scheduler_->DefaultTaskRunner();
+  if (UseThreadTaskRunnerAsDefault())
+    task_runner_ = thread_task_runner_;
+  else
+    task_runner_ = worker_scheduler_->DefaultTaskRunner();
   idle_task_runner_ = worker_scheduler_->IdleTaskRunner();
   web_scheduler_.reset(new WebSchedulerImpl(
       worker_scheduler_.get(), worker_scheduler_->IdleTaskRunner(),
@@ -80,6 +85,10 @@ void WebThreadImplForWorkerScheduler::WillDestroyCurrentMessageLoop() {
   worker_scheduler_.reset();
 }
 
+bool WebThreadImplForWorkerScheduler::UseThreadTaskRunnerAsDefault() const {
+  return false;
+}
+
 blink::PlatformThreadId WebThreadImplForWorkerScheduler::threadId() const {
   return thread_->GetThreadId();
 }