Web MIDI: fix multi-threading problem around message buffer handling

content/renderer/media/midi_message_filter.h

Index: content/renderer/media/midi_message_filter.h
diff --git a/content/renderer/media/midi_message_filter.h b/content/renderer/media/midi_message_filter.h
index cf969e9d8078401a5de3b665b4a0ddfcae4a571f..9b5ab759b23950b1f8ba1d732ff2a64ac4dcbc51 100644
--- a/content/renderer/media/midi_message_filter.h
+++ b/content/renderer/media/midi_message_filter.h
@@ -37,7 +37,7 @@ class CONTENT_EXPORT MIDIMessageFilter
 
   // A client will only be able to call this method if it has a suitable
   // output port (from addOutputPort()).
-  void SendMIDIData(int port,
+  void SendMIDIData(unsigned int port,
                     const uint8* data,
                     size_t length,
                     double timestamp);
@@ -89,7 +89,7 @@ class CONTENT_EXPORT MIDIMessageFilter
 
   void StartSessionOnIOThread(int client_id);
 
-  void SendMIDIDataOnIOThread(int port,
+  void SendMIDIDataOnIOThread(unsigned int port,
                               const std::vector<uint8>& data,
                               double timestamp);