Enforce non-negative BitstreamBuffer id

ppapi/api/dev/pp_video_dev.idl

Index: ppapi/api/dev/pp_video_dev.idl
diff --git a/ppapi/api/dev/pp_video_dev.idl b/ppapi/api/dev/pp_video_dev.idl
index 51232328261decda645424beeca51331a64e8577..936eede17765e1eb369c1d1bd1b074be1a29bb86 100644
--- a/ppapi/api/dev/pp_video_dev.idl
+++ b/ppapi/api/dev/pp_video_dev.idl
@@ -40,7 +40,8 @@ enum PP_VideoDecoder_Profile {
 [assert_size(12)]
 struct PP_VideoBitstreamBuffer_Dev {
   /**
-   * Client-specified identifier for the bitstream buffer.
+   * Client-specified identifier for the bitstream buffer. Valid values are
+   * non-negative.
    */
   int32_t id;
 
@@ -64,7 +65,7 @@ struct PP_PictureBuffer_Dev {
   /**
    * Client-specified id for the picture buffer. By using this value client can
    * keep track of the buffers it has assigned to the video decoder and how they
-   * are passed back to it.
+   * are passed back to it. Valid values are non-negative.
    */
   int32_t id;