Web MIDI: use performance.now() value if timestamp is omitted

When timestamp is omitted, it is assumed the value is 0, and handled
to be sent as soon as possible. But in Mac, the timestamp is carried
through CoreMIDI API, and it affects outgoing MIDI events which can be
handled by external applications. As a result, the events have wrong
timestamp 0, and applications may misunderstand when the event is received.
To avoid this problem, calculate right DOMHighResTimeStamp here when
the timestamp is 0, and send it as a timestamp to embedder.

BUG=341311

Review URL: https://codereview.chromium.org/166613002

git-svn-id: svn://svn.chromium.org/blink/trunk@167515 bbb929c8-8fbe-4397-9dbb-9b2b20218538