Change PPB_VideoWriter Open function to return stream_id as out param.

ppapi/cpp/video_writer.h

Index: ppapi/cpp/video_writer.h
diff --git a/ppapi/cpp/video_writer.h b/ppapi/cpp/video_writer.h
index c84091021f6f3f600d98db3af3a953cb1d1ad8b7..0b878c458c218f0d7654624a8f40c8d26b364c0f 100644
--- a/ppapi/cpp/video_writer.h
+++ b/ppapi/cpp/video_writer.h
@@ -45,13 +45,12 @@ class VideoWriter : public Resource {
 
   /// Opens a stream for writing video and associates it with the given id.
   ///
-  /// @param[in] stream_id A string uniquely identifying the stream to write to.
-  /// @param[in] callback A <code>CompletionCallback</code> to be called upon
-  /// completion of Open.
+  /// @param[in] callback A <code>CompletionCallbackWithOutput</code> to be
+  /// called upon completion of Open which writes the stream id generated by
+  /// the host.
   ///
   /// @return A return code from <code>pp_errors.h</code>.
-  int32_t Open(const std::string& stream_id,
-               const CompletionCallback& cc);
+  int32_t Open(const CompletionCallbackWithOutput<Var>& cc);
 
   /// Puts the next frame of video to the writer's stream.
   ///