Simplify LoadableTextTrack construction

HTMLTrackElement::parseAttribute() already sets kind, label and
language when the corresponding attributes change, so we can create a
TextTrack with the default values, leaving all of the logic for
parsing kind in TextTrack::setKind().

This leaves HTMLTrackElement::srclang() and label() unused, so just
use [Reflect] in the IDL for these attributes.

BUG=315510
TEST=media/track/opera/interfaces/TextTrack/kind.html,
     media/track/opera/interfaces/TextTrack/label.html,
     media/track/opera/interfaces/TextTrack/language.html,
     media/track/track-kind.html

Review URL: https://codereview.chromium.org/60423008

git-svn-id: svn://svn.chromium.org/blink/trunk@161457 bbb929c8-8fbe-4397-9dbb-9b2b20218538