Web MIDI: MIDIPort doesn't have to be ActiveDOMObject

MIDIAccess is the only class which should be ActiveDOMObject.
MIDIPort, MIDIInput, and MIDIOutput does't have to be.

BUG=163795

Review URL: https://codereview.chromium.org/23609033

git-svn-id: svn://svn.chromium.org/blink/trunk@167183 bbb929c8-8fbe-4397-9dbb-9b2b20218538