Bug 882535 - Set HTMLTrackElement Readiness State during WebVTT Parsing. r=rillian

This patch enables "LOADING", "LOADED" and "ERROR" readiness states to be
applied to the Parent HTMLTrackElement, in order to avoid test flakiness,
and to comply with the HTML5 draft
http://www.w3.org/html/wg/drafts/html/master/embedded-content-0.html#text-track-readiness-state.