Web MIDI: introduce WebMIDIClient API to request a permission

public/platform/WebMIDIAccessor.h

Index: public/platform/WebMIDIAccessor.h
diff --git a/public/platform/WebMIDIAccessor.h b/public/platform/WebMIDIAccessor.h
index 2c5a3c04019cabdd6140fec820015ecd0075ab92..4534c760a1e7892008de44c9b8a579f06efa4847 100644
--- a/public/platform/WebMIDIAccessor.h
+++ b/public/platform/WebMIDIAccessor.h
@@ -41,6 +41,7 @@ public:
 
     // |access| is set to true if MIDIOptions.sysexEnabled is true. Otherwise false.
     virtual void requestAccess(bool access) { }
+    virtual void requestAccess(const WebString& origin, bool access) { }

[abarth-chromium, 2013/07/01 17:42:21]

What do you do with the origin on the Chromium side?  Should we pass a
WebSecurityOrigin instead?  I realize that will cause a layering violation. 
Perhaps |requestAccess| should be part of the client API rather than the
platform API.  Most of the permission requests get routed through the client
rather than the platform.

     // |timeStamp| is measured in milliseconds as Web MIDI spec defines.
     virtual void sendMIDIData(unsigned portIndex, const unsigned char* data, size_t length, double timeStamp) { }
 };