Web MIDI: implement WebMIDIClientMock to emulate permission handling for testing

BUG=163795
TEST=LayoutTests/webmidi/permission.html

Review URL: https://chromiumcodereview.appspot.com/22430012

git-svn-id: svn://svn.chromium.org/blink/trunk@156143 bbb929c8-8fbe-4397-9dbb-9b2b20218538