Prevent some string copies when sending Values to WebUI.

base/json/json_writer.h

 // Copyright (c) 2012 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 BASE_JSON_JSON_WRITER_H_
 #define BASE_JSON_JSON_WRITER_H_
 
 #include <stddef.h>
 
 #include <string>
@@ skipping 19 matching lines @@
     // or appending a '.0') as long as the value is within the range of a
     // 64-bit int.
     OPTIONS_OMIT_DOUBLE_TYPE_PRESERVATION = 1 << 1,
 
     // Return a slightly nicer formatted json string (pads with whitespace to
     // help with readability).
     OPTIONS_PRETTY_PRINT = 1 << 2,
   };
 
   // Given a root node, generates a JSON string and puts it into |json|.
+  // The output string is overwritten and not appended.
+  //
   // TODO(tc): Should we generate json if it would be invalid json (e.g.,
   // |node| is not a DictionaryValue/ListValue or if there are inf/-inf float
   // values)? Return true on success and false on failure.
   static bool Write(const Value& node, std::string* json);
 
   // Same as above but with |options| which is a bunch of JSONWriter::Options
   // bitwise ORed together. Return true on success and false on failure.
   static bool WriteWithOptions(const Value& node,
                                int options,
                                std::string* json);
@@ skipping 14 matching lines @@
 
   // Where we write JSON data as we generate it.
   std::string* json_string_;
 
   DISALLOW_COPY_AND_ASSIGN(JSONWriter);
 };
 
 }  // namespace base
 
 #endif  // BASE_JSON_JSON_WRITER_H_