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 13d23d7fe8ea8dd893d4b3ddcc1d630d44cc0f03..345ecef9690b20b189084f190bcf708e2f0ead33 100644
--- a/components/scheduler/child/webthread_impl_for_worker_scheduler.cc
+++ b/components/scheduler/child/webthread_impl_for_worker_scheduler.cc
@@ -40,13 +40,15 @@ void WebThreadImplForWorkerScheduler::Init() {
 }
 
 WebThreadImplForWorkerScheduler::~WebThreadImplForWorkerScheduler() {
-  base::WaitableEvent completion(false, false);
-  // Restore the original task runner so that the thread can tear itself down.
-  thread_task_runner_->PostTask(
-      FROM_HERE,
-      base::Bind(&WebThreadImplForWorkerScheduler::RestoreTaskRunnerOnThread,
-                 base::Unretained(this), &completion));
-  completion.Wait();
+  if (task_runner_delegate_) {
+    base::WaitableEvent completion(false, false);
+    // Restore the original task runner so that the thread can tear itself down.
+    thread_task_runner_->PostTask(
+        FROM_HERE,
+        base::Bind(&WebThreadImplForWorkerScheduler::RestoreTaskRunnerOnThread,
+                   base::Unretained(this), &completion));
+    completion.Wait();
+  }
   thread_->Stop();
 }
 
@@ -54,14 +56,9 @@ void WebThreadImplForWorkerScheduler::InitOnThread(
     base::WaitableEvent* completion) {
   // TODO(alexclarke): Do we need to unify virtual time for workers and the
   // main thread?
-  task_runner_delegate_ = SchedulerTqmDelegateImpl::Create(
-      thread_->message_loop(), make_scoped_ptr(new base::DefaultTickClock()));
-  worker_scheduler_ = WorkerScheduler::Create(task_runner_delegate_);
+  worker_scheduler_ = CreateWorkerScheduler();
   worker_scheduler_->Init();
-  if (UseThreadTaskRunnerAsDefault())
-    task_runner_ = thread_task_runner_;
-  else
-    task_runner_ = worker_scheduler_->DefaultTaskRunner();
+  task_runner_ = worker_scheduler_->DefaultTaskRunner();
   idle_task_runner_ = worker_scheduler_->IdleTaskRunner();
   web_scheduler_.reset(new WebSchedulerImpl(
       worker_scheduler_.get(), worker_scheduler_->IdleTaskRunner(),
@@ -85,8 +82,11 @@ void WebThreadImplForWorkerScheduler::WillDestroyCurrentMessageLoop() {
   worker_scheduler_.reset();
 }
 
-bool WebThreadImplForWorkerScheduler::UseThreadTaskRunnerAsDefault() const {
-  return false;
+scoped_ptr<scheduler::WorkerScheduler>
+WebThreadImplForWorkerScheduler::CreateWorkerScheduler() {
+  task_runner_delegate_ = SchedulerTqmDelegateImpl::Create(
+      thread_->message_loop(), make_scoped_ptr(new base::DefaultTickClock()));
+  return WorkerScheduler::Create(task_runner_delegate_);
 }
 
 blink::PlatformThreadId WebThreadImplForWorkerScheduler::threadId() const {