Revert of Move tracing namespace from base::debug to base::trace_event.

cc/resources/zero_copy_tile_task_worker_pool.h

 // Copyright 2013 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_RESOURCES_ZERO_COPY_TILE_TASK_WORKER_POOL_H_
 #define CC_RESOURCES_ZERO_COPY_TILE_TASK_WORKER_POOL_H_
 
 #include "base/memory/weak_ptr.h"
 #include "base/values.h"
 #include "cc/resources/tile_task_runner.h"
 #include "cc/resources/tile_task_worker_pool.h"
 
 namespace base {
-namespace trace_event {
+namespace debug {
 class ConvertableToTraceFormat;
 }
-
-// TODO(ssid): remove these aliases after the tracing clients are moved to the
-// new trace_event namespace. See crbug.com/451032. ETA: March 2015
-namespace debug {
-using ::base::trace_event::ConvertableToTraceFormat;
 }
-}  // namespace base
 
 namespace cc {
 class ResourceProvider;
 
 class CC_EXPORT ZeroCopyTileTaskWorkerPool : public TileTaskWorkerPool,
                                              public TileTaskRunner,
                                              public TileTaskClient {
  public:
   ~ZeroCopyTileTaskWorkerPool() override;
 
@@ skipping 43 matching lines @@
 
   base::WeakPtrFactory<ZeroCopyTileTaskWorkerPool>
       task_set_finished_weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ZeroCopyTileTaskWorkerPool);
 };
 
 }  // namespace cc
 
 #endif  // CC_RESOURCES_ZERO_COPY_TILE_TASK_WORKER_POOL_H_