Change PPB_VideoWriter Open function to return stream_id as out param.

ppapi/cpp/video_reader.cc

Index: ppapi/cpp/video_reader.cc
diff --git a/ppapi/cpp/video_reader.cc b/ppapi/cpp/video_reader.cc
index ac7fe79adf7daf04db30ecae4d5fce54c0165c4f..9ae83bbd662539b33d95af66e541959fad7f91f4 100644
--- a/ppapi/cpp/video_reader.cc
+++ b/ppapi/cpp/video_reader.cc
@@ -42,14 +42,13 @@ VideoReader::VideoReader(PassRef, PP_Resource resource)
     : Resource(PASS_REF, resource) {
 }
 
-int32_t VideoReader::Open(const std::string& stream_id,
+int32_t VideoReader::Open(const Var& stream_id,
                           const CompletionCallback& cc) {
   if (has_interface<PPB_VideoReader_0_1>()) {
-    Var id(stream_id);
     int32_t result =
         get_interface<PPB_VideoReader_0_1>()->Open(
             pp_resource(),
-            id.pp_var(), cc.pp_completion_callback());
+            stream_id.pp_var(), cc.pp_completion_callback());
     return result;
   }
   return cc.MayForce(PP_ERROR_NOINTERFACE);