compositor worker: Use a WebThread for the compositor thread.

components/scheduler/child/webthread_impl_for_worker_scheduler.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/scheduler/child/webthread_impl_for_worker_scheduler.h"
 
 #include "base/bind.h"
 #include "base/location.h"
 #include "base/single_thread_task_runner.h"
 #include "base/synchronization/waitable_event.h"
@@ skipping 22 matching lines @@
 
 void WebThreadImplForWorkerScheduler::Init() {
   base::WaitableEvent completion(false, false);
   thread_task_runner_->PostTask(
       FROM_HERE, base::Bind(&WebThreadImplForWorkerScheduler::InitOnThread,
                             base::Unretained(this), &completion));
   completion.Wait();
 }
 
 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();
 }
 
 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(),
       worker_scheduler_->DefaultTaskRunner(),
       worker_scheduler_->DefaultTaskRunner()));
   base::MessageLoop::current()->AddDestructionObserver(this);
   web_task_runner_ = make_scoped_ptr(new WebTaskRunnerImpl(task_runner_));
   completion->Signal();
 }
 
 void WebThreadImplForWorkerScheduler::RestoreTaskRunnerOnThread(
     base::WaitableEvent* completion) {
   task_runner_delegate_->RestoreDefaultTaskRunner();
   completion->Signal();
 }
 
 void WebThreadImplForWorkerScheduler::WillDestroyCurrentMessageLoop() {
   task_runner_ = nullptr;
   idle_task_runner_ = nullptr;
   web_scheduler_.reset();
   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 {
   return thread_->GetThreadId();
 }
 
 blink::WebScheduler* WebThreadImplForWorkerScheduler::scheduler() const {
   return web_scheduler_.get();
 }
 
@@ skipping 15 matching lines @@
     base::MessageLoop::TaskObserver* observer) {
   worker_scheduler_->AddTaskObserver(observer);
 }
 
 void WebThreadImplForWorkerScheduler::RemoveTaskObserverInternal(
     base::MessageLoop::TaskObserver* observer) {
   worker_scheduler_->RemoveTaskObserver(observer);
 }
 
 }  // namespace scheduler