Add a deprecated stub for WKPreferencesSetMultithreadedWebGLEnabled (and Get)

Add a deprecated stub for WKPreferencesSetMultithreadedWebGLEnabled (and Get)
https://bugs.webkit.org/show_bug.cgi?id=137071

Reviewed by Brent Fulgham.

Put in stubs for WKPreferencesSetMultithreadedWebGLEnabled and
WKPreferencesGetMultithreadedWebGLEnabled. These were removed in
r173776 but are still needed for nightly builds to link
(or will be once Yosemite nightlies are available).

* Shared/API/c/WKDeprecatedFunctions.cpp:
(WKPreferencesSetMultithreadedWebGLEnabled):
(WKPreferencesGetMultithreadedWebGLEnabled):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@173927 268f45cc-cd09-0410-ab3c-d52691b4dbfc