Web MIDI: remove old platform API

Remove obsoleted APIs, requestAccess of WebMIDIAccessor, and
didAllowAccess and didBlockAccess of WebMIDIAccessorClient.

TEST=none 
BUG=163795

Review URL: https://chromiumcodereview.appspot.com/18260005

git-svn-id: svn://svn.chromium.org/blink/trunk@155004 bbb929c8-8fbe-4397-9dbb-9b2b20218538