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

cc/layers/layer_client.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_LAYERS_LAYER_CLIENT_H_
 #define CC_LAYERS_LAYER_CLIENT_H_
 
 #include <string>
 
 #include "base/memory/ref_counted.h"
 #include "cc/base/cc_export.h"
 
 namespace base {
-namespace debug {
+namespace trace_event {
 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 CC_EXPORT LayerClient {
  public:
   // Returns a pointer to a debug info object, if one has been computed.
   // If not, returns nullptr.
   // If the returned pointer is non-nullptr, the caller takes
   // ownership of the pointer.
   virtual scoped_refptr<base::debug::ConvertableToTraceFormat>
       TakeDebugInfo() = 0;
 
  protected:
   virtual ~LayerClient() {}
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_LAYER_CLIENT_H_