Make TracedValue lower overhead.

base/json/json_writer.h

Index: base/json/json_writer.h
diff --git a/base/json/json_writer.h b/base/json/json_writer.h
index 94052c8034321dd28e6adf4699029e287eb93a6f..bd937f13f615ac07ba676037f5e377f1e06fa50c 100644
--- a/base/json/json_writer.h
+++ b/base/json/json_writer.h
@@ -48,6 +48,10 @@ class BASE_EXPORT JSONWriter {
   static void WriteWithOptions(const Value* const node, int options,
                                std::string* json);
 
+  // Returns a JSON compliant string representation of a double value.
+  static std::string DoubleToFormattedString(double value,
+      bool omit_double_type_preservation);
+
   // A static, constant JSON string representing an empty array.  Useful
   // for empty JSON argument passing.
   static const char* kEmptyArray;