Support padding, margin and border for internal UA cue styling
https://bugs.webkit.org/show_bug.cgi?id=110703

Reviewed by Eric Carlson.

Source/WebCore:

For some particular user agent styling this allows the possibility
of making the window around the cue text bigger to match some user
styles (see CaptionUserPreferencesMac::captionsStyleSheetOverride).

These properties *cannot* be set through by using the ::cue
pseudo-element and, as specified, are used only internally.

Test: media/track/track-cue-rendering-with-padding.html

* css/mediaControls.css:
(video::-webkit-media-text-track-display): Set the CSS box model
to include in the specified width or height the values of
padding / margin / border by using -webkit-box-sizing and avoid
overflow over 100% width because of having these properties set.
* rendering/RenderTextTrackCue.cpp:
(WebCore::RenderTextTrackCue::isOutside): To not interfere with
the regular WebVTT positioning algorithm, the check is done for
the absolute content box.
(WebCore::RenderTextTrackCue::repositionCueSnapToLinesSet): Added
an extra adjustment step to accomodate vertical padding (and not
overflow the cue container)

LayoutTests:

* media/track/track-cue-rendering-with-padding-expected.txt: Added.
* media/track/track-cue-rendering-with-padding.html: Added.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@144443 268f45cc-cd09-0410-ab3c-d52691b4dbfc