Unreviewed, rolling out r176566.
https://bugs.webkit.org/show_bug.cgi?id=139124

It broke the GTK performance tests. (Requested by clopez on
#webkit).

Reverted changeset:

"[GTK] Use GMainLoopSource in WebKitTestRunner"
https://bugs.webkit.org/show_bug.cgi?id=138831
http://trac.webkit.org/changeset/176566

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@176591 268f45cc-cd09-0410-ab3c-d52691b4dbfc