Prevent some string copies when sending Values to WebUI.

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 ef433414090aa5aea7f889193c86ead13bb1c4fd..57cb8c16a29031d924be9f08e8a918ec826dba88 100644
--- a/base/json/json_writer.h
+++ b/base/json/json_writer.h
@@ -37,6 +37,8 @@ class BASE_EXPORT JSONWriter {
   };
 
   // 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.