Add builders for tracing event's structural arguments

cc/resources/raster_mode.cc

 // 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.
 
 #include "cc/resources/raster_mode.h"
 
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/values.h"
 
 namespace cc {
 
-scoped_ptr<base::Value> RasterModeAsValue(RasterMode raster_mode) {
+std::string RasterModeToString(RasterMode raster_mode) {
   switch (raster_mode) {
     case HIGH_QUALITY_RASTER_MODE:
-      return scoped_ptr<base::Value>(
-          new base::StringValue("HIGH_QUALITY_RASTER_MODE"));
+      return "HIGH_QUALITY_RASTER_MODE";
     case LOW_QUALITY_RASTER_MODE:
-      return scoped_ptr<base::Value>(
-          new base::StringValue("LOW_QUALITY_RASTER_MODE"));
+      return "LOW_QUALITY_RASTER_MODE";
     default:
       NOTREACHED() << "Unrecognized RasterMode value " << raster_mode;
-      return scoped_ptr<base::Value>(
-          new base::StringValue("<unknown RasterMode value>"));
+      return "<unknown RasterMode value>";
   }
 }
 
 }  // namespace cc