[GTK] Rollout r170529 due to ~10% performance regression on the
perf test Animation/balls.
https://bugs.webkit.org/show_bug.cgi?id=134972

Reviewed by Martin Robinson.

Reverted changeset:
"Increase priority on SharedTimer source."
https://trac.webkit.org/r170529


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@171343 268f45cc-cd09-0410-ab3c-d52691b4dbfc