Disable EXT_draw_buffers WebGL extension on Mac
https://bugs.webkit.org/show_bug.cgi?id=112486

Reviewed by Kenneth Russell.

* html/canvas/EXTDrawBuffers.cpp:
(WebCore::EXTDrawBuffers::supported):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@145972 268f45cc-cd09-0410-ab3c-d52691b4dbfc