compositor worker: Use a WebThread for the compositor thread.

components/scheduler/renderer/renderer_scheduler_impl.h

 // Copyright 2014 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.
 
 #ifndef COMPONENTS_SCHEDULER_RENDERER_RENDERER_SCHEDULER_IMPL_H_
 #define COMPONENTS_SCHEDULER_RENDERER_RENDERER_SCHEDULER_IMPL_H_
 
 #include "base/atomicops.h"
 #include "base/macros.h"
 #include "base/synchronization/lock.h"
@@ skipping 23 matching lines @@
     : public RendererScheduler,
       public IdleHelper::Delegate,
       public SchedulerHelper::Observer,
       public RenderWidgetSignals::Observer {
  public:
   RendererSchedulerImpl(scoped_refptr<SchedulerTqmDelegate> main_task_runner);
   ~RendererSchedulerImpl() override;
 
   // RendererScheduler implementation:
   scoped_ptr<blink::WebThread> CreateMainThread() override;
-  scoped_refptr<TaskQueue> DefaultTaskRunner() override;
+  scoped_refptr<base::SingleThreadTaskRunner> DefaultTaskRunner() override;
   scoped_refptr<SingleThreadIdleTaskRunner> IdleTaskRunner() override;
   scoped_refptr<base::SingleThreadTaskRunner> CompositorTaskRunner() override;
   scoped_refptr<base::SingleThreadTaskRunner> LoadingTaskRunner() override;
   scoped_refptr<TaskQueue> TimerTaskRunner() override;
   scoped_refptr<TaskQueue> NewLoadingTaskRunner(const char* name) override;
   scoped_refptr<TaskQueue> NewTimerTaskRunner(const char* name) override;
   scoped_ptr<RenderWidgetSchedulingState> NewRenderWidgetSchedulingState()
       override;
   void WillBeginFrame(const cc::BeginFrameArgs& args) override;
   void BeginFrameNotExpectedSoon() override;
@@ skipping 307 matching lines @@
 
   PollableThreadSafeFlag policy_may_need_update_;
   base::WeakPtrFactory<RendererSchedulerImpl> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(RendererSchedulerImpl);
 };
 
 }  // namespace scheduler
 
 #endif  // COMPONENTS_SCHEDULER_RENDERER_RENDERER_SCHEDULER_IMPL_H_