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

public/platform/WebMIDIAccessor.h

Index: public/platform/WebMIDIAccessor.h
diff --git a/public/platform/WebMIDIAccessor.h b/public/platform/WebMIDIAccessor.h
index ace32ed8826490ef534b8e439e22e974ac4a7f49..a31cfda7c842b869a9ed1767cbf6ceff4a3660c2 100644
--- a/public/platform/WebMIDIAccessor.h
+++ b/public/platform/WebMIDIAccessor.h
@@ -40,8 +40,11 @@ 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