Use a separate backdrop element to allow cues to have highlight and background color

Use a separate backdrop element to allow cues to have highlight and background color
https://bugs.webkit.org/show_bug.cgi?id=134821
<rdar://problem/15999721>

Reviewed by Eric Carlson.

Source/WebCore:
Add a new <div> element wrapping the existing cue <span>. This allows
us to have a highlight on the cue (in the <span> background), as well
as an overall background color.

* Modules/mediacontrols/mediaControlsApple.css:
(video::-webkit-media-text-track-display-backdrop): New markup for
the backdrop element of the caption.
* html/track/VTTCue.cpp:
(WebCore::VTTCue::cueBackdropShadowPseudoId): Added to
allow user customization of the cue backdrop.
(WebCore::VTTCue::initialize): Rename the old "m_cueBackgroundBox" to
"m_cueHighlightBox" and add a new "m_cueBackdropBox" member.
(WebCore::VTTCue::updateDisplayTree): Update for m_cueHighlightBox.
(WebCore::VTTCue::getDisplayTree): Make m_cueHighlightBox a child
of the new m_cueBackdropBox element, and add m_cueBackdropBox to
the display tree.
* html/track/VTTCue.h:
(WebCore::VTTCue::element):
* page/CaptionUserPreferencesMediaAF.cpp:
(WebCore::CaptionUserPreferencesMediaAF::setInterestedInCaptionPreferenceChanges):
Fix for missing caption style updates. Even if we are already
listening for caption changes, we still want to update the new
instance's style sheet to match.
(WebCore::CaptionUserPreferencesMediaAF::captionsStyleSheetOverride):
* rendering/RenderVTTCue.cpp:
(WebCore::RenderVTTCue::initializeLayoutParameters): Take the new
<div> into consideration when looking for the Cue text element.

LayoutTests:
Updated tests for new formatting logic.

* platform/mac/media/track/track-cue-rendering-horizontal-expected.png:
* platform/mac/media/track/track-cue-rendering-horizontal-expected.txt:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@171004 268f45cc-cd09-0410-ab3c-d52691b4dbfc