Remove WebInbandTextTrack::isDefault()

The spec only mentions the default attribute on track elements
(out-of-band text tracks), in-band tracks don't have this
concept.

Also, because InbandTextTrack doesn't actually implement
isDefault(), this is all dead code.

The Chromium implementation (WebInbandTextTrackImpl) always
returns false and can be removed after a Blink roll.

Also remove HTMLTrackElement::isDefault() and setIsDefault(),
which are dead code because [Reflect] is used in the IDL.

BUG=315270

Review URL: https://codereview.chromium.org/60633004

git-svn-id: svn://svn.chromium.org/blink/trunk@161396 bbb929c8-8fbe-4397-9dbb-9b2b20218538