Upstream USE(IOSURFACE) from the iOS port
https://bugs.webkit.org/show_bug.cgi?id=124814

Reviewed by Sam Weinig.

Some code guarded by USE(IOSURFACE) was recently upstreamed,
but not the definition of WTF_USE_IOSURFACE itself.

* wtf/Platform.h:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@159735 268f45cc-cd09-0410-ab3c-d52691b4dbfc