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

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
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=201860

[Takashi Toyoshima, 2015-09-04 07:03:42]

toyoshim@chromium.org changed reviewers:
+ tkent@chromium.org

[Takashi Toyoshima, 2015-09-04 07:03:51]

PTAL

[tkent, 2015-09-06 23:50:12]

lgtm

[Takashi Toyoshima, 2015-09-07 06:06:35]

The CQ bit was checked by toyoshim@chromium.org

[commit-bot: I haz the power, 2015-09-07 06:06:45]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1327993003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1327993003/1

[commit-bot: I haz the power, 2015-09-07 06:51:51]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201860