Bug 882664 - Part 1: TextTrack should hold a ref to its TrackElement, if any. r=rillian
Bug 882664 - Part 2: Perform sorting on TextTrackLists. r=rillian

- This implements sorting on the MediaElement's list of TextTracks as
described in: http://www.whatwg.org/specs/web-apps/current-work/multipage/the-video-element.html#text-track-model
This is important as automatically positioned cues will have a line
position that is relative to their TextTrack's position in the
MediaElement's list of TextTracks.
- Added a TextTrackSource enum which all TextTracks have. This helps
distinguish where a TextTrack came from.
Bug 970363 - Make PeekOffset* methods return continue reason. r=roc
Bug 970363 - Fix selection moving over readonly elements in contenteditble blocks. r=roc
Bug 982302 - Braces are missing in nsBaseURLParser::ParseURL. r=mcmanus
Bug 966564 - Make logging more clear when processing an unwrapped indication. r=drno