Use reference when copying strings into mapbuffer

Summary:
This method can produce unnecessary copy of the string, replacing it with reference to optimize.

Changelog: [Internal]

Reviewed By: mdvacca

Differential Revision: D33607765

fbshipit-source-id: e04eddffec063c0c8e173bfdf690e7b8e1050525

ReactCommon/react/renderer/mapbuffer/MapBufferBuilder.cpp

@@ -125,7 +125,7 @@ void MapBufferBuilder::ensureDynamicDataSpace(int32_t size) {
   }
 }
 
-void MapBufferBuilder::putString(Key key, std::string value) {
+void MapBufferBuilder::putString(Key key, std::string const &value) {
   int32_t strLength = static_cast<int32_t>(value.length());
   const char *cstring = getCstring(&value);
 

ReactCommon/react/renderer/mapbuffer/MapBufferBuilder.h

@@ -82,7 +82,7 @@ class MapBufferBuilder {
 
   void putNull(Key key);
 
-  void putString(Key key, std::string value);
+  void putString(Key key, std::string const &value);
 
   void putMapBuffer(Key key, MapBuffer &map);