https://bugs.webkit.org/show_bug.cgi?id=114460

Reviewed by Jer Noble.

Source/WebCore: 

Test: media/track/track-forced-subtitles.html

* html/HTMLMediaElement.cpp:
(WebCore::HTMLMediaElement::configureTextTrackGroup): Enable a forced track if nothing else is enabled.
(WebCore::HTMLMediaElement::configureTextTracks): Include forced tracks with captions+subtitles.

* html/track/InbandTextTrack.cpp:
(WebCore::InbandTextTrack::InbandTextTrack): Deal with kind=forced.

* html/track/TextTrack.cpp:
(WebCore::TextTrack::forcedKeyword): New keyword.
(WebCore::TextTrack::isValidKindKeyword): Include forced.
(WebCore::TextTrack::platformTextTrack): Ditto.
* html/track/TextTrack.h:

* page/CaptionUserPreferences.cpp:
(WebCore::CaptionUserPreferences::primaryAudioTrackLanguageOverride): New override to support testing.
* page/CaptionUserPreferences.h:

* page/CaptionUserPreferencesMac.mm:
(WebCore::CaptionUserPreferencesMac::textTrackSelectionScore): Support forced tracks.
* platform/graphi...