Web MIDI: update blink API for open(), clear(), and close() backends

public/platform/WebMIDIAccessor.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 22 matching lines @@
 
 #include "WebString.h"
 
 namespace blink {
 
 class WebMIDIAccessor {
 public:
     virtual ~WebMIDIAccessor() { }
 
     virtual void startSession() { }
+    virtual void open(unsigned portIndex) { }
     // |timeStamp| is measured in milliseconds as Web MIDI spec defines.
     virtual void sendMIDIData(unsigned portIndex, const unsigned char* data, size_t length, double timeStamp) { }
+    virtual void clear(unsigned portIndex) { }
+    virtual void close(unsigned portIndex) { }
 };
 
 } // namespace blink
 
 #endif // WebMIDIAccessor_h