Add builders for tracing event's structural arguments

cc/resources/raster_mode.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_RASTER_MODE_H_
 #define CC_RESOURCES_RASTER_MODE_H_
 
+#include <string>
+
 #include "base/memory/scoped_ptr.h"
 
 namespace base {
 class Value;
 }
 
 namespace cc {
 
 // Note that the order of these matters, from "better" to "worse" in terms of
 // quality.
 enum RasterMode {
   HIGH_QUALITY_RASTER_MODE = 0,
   LOW_QUALITY_RASTER_MODE = 1,
   NUM_RASTER_MODES = 2
 };
 
-scoped_ptr<base::Value> RasterModeAsValue(RasterMode mode);
+std::string RasterModeToString(RasterMode mode);
 
 }  // namespace cc
 
 #endif  // CC_RESOURCES_RASTER_MODE_H_