Change PPB_VideoWriter Open function to return stream_id as out param.

ppapi/api/ppb_video_writer.idl

 /* Copyright (c) 2013 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.
  */
 
 /**
  * This file defines the <code>PPB_VideoWriter</code> struct for a video writer
  * resource.
  */
 
@@ skipping 26 matching lines @@
    * @return A <code>PP_Bool</code> with <code>PP_TRUE</code> if the given
    * resource is a video writer or <code>PP_FALSE</code> otherwise.
    */
   PP_Bool IsVideoWriter([in] PP_Resource resource);
 
   /**
    * Opens a video stream with the given id for writing.
    *
    * @param[in] writer A <code>PP_Resource</code> corresponding to a video
    * writer resource.
-   * @param[in] stream_id A <code>PP_Var</code> holding a string uniquely
-   * identifying the stream. This string is application defined.
+   * @param[out] stream_id A <code>PP_Var</code> holding a string uniquely
+   * identifying the stream. This string is generated by the host.
    * @param[in] callback A <code>PP_CompletionCallback</code> to be called upon
    * completion of Open().
    *
    * @return An int32_t containing an error code from <code>pp_errors.h</code>.
    * Returns PP_ERROR_BADRESOURCE if writer isn't a valid video writer.
    * Returns PP_ERROR_INPROGRESS if the writer has already opened a stream.
    */
   int32_t Open([in] PP_Resource writer,
-               [in] PP_Var stream_id,
+               [out] PP_Var stream_id,
                [in] PP_CompletionCallback callback);
 
   /**
    * Puts a frame of video to the writer's open stream.
    *
    * After this call, you should take care to release your references to the
    * image embedded in the video frame. If you paint to the image after
    * PutFrame(), there is the possibility of artifacts because the browser may
    * still be copying the frame to the stream.
    *
@@ skipping 12 matching lines @@
 
   /**
    * Closes the writer's video stream.
    *
    * @param[in] writer A <code>PP_Resource</code> corresponding to a video
    * writer resource.
    */
   void Close([in] PP_Resource writer);
 };