Revert of TaskGraphRunner Group support

cc/raster/single_thread_task_graph_runner.h

 // 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.
 
 #ifndef CC_RASTER_SINGLE_THREAD_TASK_GRAPH_RUNNER_H_
 #define CC_RASTER_SINGLE_THREAD_TASK_GRAPH_RUNNER_H_
 
 #include "base/memory/scoped_ptr.h"
 #include "base/synchronization/condition_variable.h"
 #include "base/threading/simple_thread.h"
@@ skipping 22 matching lines @@
                              Task::Vector* completed_tasks) override;
 
   // Overridden from base::DelegateSimpleThread::Delegate:
   void Run() override;
 
   void Start(const std::string& thread_name,
              const base::SimpleThread::Options& thread_options);
   void Shutdown();
 
  private:
-  // Returns true if there was a task to run.
-  bool RunTaskWithLockAcquired();
+  void RunTaskWithLockAcquired();
 
   scoped_ptr<base::SimpleThread> thread_;
 
   // Lock to exclusively access all the following members that are used to
   // implement the TaskRunner interfaces.
   base::Lock lock_;
 
   // Stores the actual tasks to be run by this runner, sorted by priority.
   TaskGraphWorkQueue work_queue_;
 
   // Condition variable that is waited on by Run() until new tasks are ready to
   // run or shutdown starts.
   base::ConditionVariable has_ready_to_run_tasks_cv_;
 
   // Condition variable that is waited on by origin threads until a namespace
   // has finished running all associated tasks.
   base::ConditionVariable has_namespaces_with_finished_running_tasks_cv_;
 
   // Set during shutdown. Tells Run() to return when no more tasks are pending.
   bool shutdown_;
 };
 
 }  // namespace cc
 
 #endif  // CC_RASTER_SINGLE_THREAD_TASK_GRAPH_RUNNER_H_