TextTrackCue should support empty content
https://bugs.webkit.org/show_bug.cgi?id=115821

Reviewed by Jer Noble.

Source/WebCore:

Test: media/track/track-cue-empty-text-crash.html

* html/shadow/MediaControlElements.cpp:
(WebCore::MediaControlTextTrackContainerElement::updateDisplay): Don't bother trying to render
    cues with no content.

* html/track/TextTrackCue.cpp:
(WebCore::TextTrackCue::getCueAsHTML): Return early if there isn't a node tree.
(WebCore::TextTrackCue::createCueRenderingTree): Ditto.
(WebCore::TextTrackCue::determineTextDirection): Ditto.
(WebCore::TextTrackCue::updateDisplayTree): Return early if there isn't a cue rendering tree.

LayoutTests:

* media/track/track-cue-empty-text-crash-expected.txt: Added.
* media/track/track-cue-empty-text-crash.html: Added.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149766 268f45cc-cd09-0410-ab3c-d52691b4dbfc