Make WebMIDI use blink Promise.

Use Blink Promise instead of MIDIAccessPromise.
BUG=320984
R=toyoshim@chromium.org

Review URL: https://codereview.chromium.org/77773003

git-svn-id: svn://svn.chromium.org/blink/trunk@169727 bbb929c8-8fbe-4397-9dbb-9b2b20218538