Revert of TaskGraphRunner Group support

cc/raster/synchronous_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_SYNCHRONOUS_TASK_GRAPH_RUNNER_H_
 #define CC_RASTER_SYNCHRONOUS_TASK_GRAPH_RUNNER_H_
 
 #include "cc/raster/task_graph_runner.h"
 #include "cc/raster/task_graph_work_queue.h"
 
@@ skipping 10 matching lines @@
   NamespaceToken GetNamespaceToken() override;
   void ScheduleTasks(NamespaceToken token, TaskGraph* graph) override;
   void WaitForTasksToFinishRunning(NamespaceToken token) override;
   void CollectCompletedTasks(NamespaceToken token,
                              Task::Vector* completed_tasks) override;
 
   // Runs all pending tasks from all namespaces.
   void RunUntilIdle();
 
  private:
-  // Returns true if there was a task to run.
-  bool RunTask();
+  void RunTask();
 
   // Stores the actual tasks to be run, sorted by priority.
   TaskGraphWorkQueue work_queue_;
 };
 
 }  // namespace cc
 
 #endif  // CC_RASTER_SYNCHRONOUS_TASK_GRAPH_RUNNER_H_