[Mac] Do not assume MediaAccessibility framework is installed
https://bugs.webkit.org/show_bug.cgi?id=109365

Reviewed by Sam Weinig.

* page/CaptionUserPreferencesMac.h:
* page/CaptionUserPreferencesMac.mm:
(WebCore::CaptionUserPreferencesMac::userPrefersCaptions): Call the base class if the framework
    is not available.
(WebCore::CaptionUserPreferencesMac::setUserPrefersCaptions): Ditto.
(WebCore::CaptionUserPreferencesMac::userHasCaptionPreferences): Ditto.
(WebCore::CaptionUserPreferencesMac::registerForCaptionPreferencesChangedCallbacks): Ditto.
(WebCore::CaptionUserPreferencesMac::unregisterForCaptionPreferencesChangedCallbacks): Ditto.
(WebCore::CaptionUserPreferencesMac::captionsStyleSheetOverride): Ditto.
(WebCore::CaptionUserPreferencesMac::captionFontSizeScale): Ditto.
(WebCore::CaptionUserPreferencesMac::setPreferredLanguage): Ditto.
(WebCore::CaptionUserPreferencesMac::preferredLanguages): Ditto.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@142383 268f45cc-cd09-0410-ab3c-d52691b4dbfc