webmdec: quiet -Wshorten-64-to-32 warning

track->GetNumber() will fit in an int in well-behaved files

cherry-picked from libvpx:
c79665d0a webmdec: quiet -Wshorten-64-to-32 warning

Change-Id: I344a7dd05d04daf3df2d67358ea69f8014a03a5b

webmdec.cc

@@ -90,7 +90,7 @@ int file_is_webm(struct WebmInputContext *webm_ctx,
     const mkvparser::Track *const track = tracks->GetTrackByIndex(i);
     if (track->GetType() == mkvparser::Track::kVideo) {
       video_track = static_cast<const mkvparser::VideoTrack *>(track);
-      webm_ctx->video_track_index = track->GetNumber();
+      webm_ctx->video_track_index = static_cast<int>(track->GetNumber());
       break;
     }
   }