https://bugs.webkit.org/show_bug.cgi?id=93779

Reviewed by Philippe Normand.

Compute the position of a text track relative to rendered tracks. This is needed to position
cues correctly when there is more than one text track.

Source/WebCore: 

Test: media/track/track-cue-container-rendering-position.html

* html/track/TextTrack.cpp:
(WebCore::TextTrack::invalidateTrackIndex): Invalidate both cached track indices.
(WebCore::TextTrack::trackIndexRelativeToRenderedTracks): Return the index of the track relative
    to other rendered tracks.
* html/track/TextTrack.h:

* html/track/TextTrackCue.cpp:
(WebCore::TextTrackCue::calculateComputedLinePosition): Use trackIndexRelativeToRenderedTracks()
    instead of trackIndex() so cues are positioned correctly.

* html/track/TextTrackList.cpp:
(TextTrackList::getTrackIndex): Change return type from unsigned to int.
(TextTrackList::getTrackIndexRelativeToRenderedTracks): New.
* html/track/TextTrackList.h:

LayoutTests: 

* media/track/track-cue-container-rendering-position-expected.txt: Added.
* media/track/track-cue-container-rendering-position.html: Added.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@134783 268f45cc-cd09-0410-ab3c-d52691b4dbfc