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

To implement MIDIPort#open(), #clear(), and #close(), let's add new
interfaces to implement backends for them.

Backend will be implemented in chromium side soon. Once it gets ready,
I'll update Web MIDI IDLs and modules/webmidi implementations to use
these interface to be up to date.

BUG=471793, 471798

Review URL: https://codereview.chromium.org/1327993003

git-svn-id: svn://svn.chromium.org/blink/trunk@201860 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/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