Chromium Mac fails to build after r146995 because the new NSFont category
may conflict with the system. Add the WebCoreTheme category to the whitelist.

Unreviewed.

* WebCore.gyp/WebCore.gyp:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@147012 268f45cc-cd09-0410-ab3c-d52691b4dbfc