Add builders for tracing event's structural arguments

cc/resources/raster_mode.cc

Index: cc/resources/raster_mode.cc
diff --git a/cc/resources/raster_mode.cc b/cc/resources/raster_mode.cc
index 5da8ee76e9b229677d05f5e9ad976a947ec19195..d67fdd850f445240fd81045fc8addb01842a23f3 100644
--- a/cc/resources/raster_mode.cc
+++ b/cc/resources/raster_mode.cc
@@ -10,18 +10,15 @@
 
 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>";
   }
 }