Change PPB_VideoWriter Open function to return stream_id as out param.

ppapi/api/ppb_video_writer.idl

Index: ppapi/api/ppb_video_writer.idl
diff --git a/ppapi/api/ppb_video_writer.idl b/ppapi/api/ppb_video_writer.idl
index e1a77573d9315fc3d0821fe58767aa70d2b8fba8..cf92f3e530bb84bc748455d09b2e8d65807fa126 100644
--- a/ppapi/api/ppb_video_writer.idl
+++ b/ppapi/api/ppb_video_writer.idl
@@ -44,8 +44,8 @@ interface PPB_VideoWriter {
    *
    * @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().
    *
@@ -54,7 +54,7 @@ interface PPB_VideoWriter {
    * 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);
 
   /**