Web MIDI: update platform API to receive startSession result

OS or other dependent systems may return an error when Web MIDI is going
to initialize the MIDI systems on starting a session.
The error should be exposed in JavaScript API, as an InvalidStateError
DOMError.

TEST=Layouttests/webmidi
BUG=260315

Review URL: https://chromiumcodereview.appspot.com/23376004

git-svn-id: svn://svn.chromium.org/blink/trunk@156621 bbb929c8-8fbe-4397-9dbb-9b2b20218538