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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