MIDIAccessPromise cleanup

- Stop using {un}setPendingActivity to avoid counting mismatch
- Implement ActiveDOMObject::stop()

BUG=N/A
TEST=Layouttests/webmidi

Review URL: https://codereview.chromium.org/39233002

git-svn-id: svn://svn.chromium.org/blink/trunk@160436 bbb929c8-8fbe-4397-9dbb-9b2b20218538