BUILD FIX (r146704): Settings::setHiddenPageDOMTimerThrottlingEnabled() isn't always built
<http://webkit.org/b/112308>

* WebCore.exp.in: Export
Settings::setHiddenPageDOMTimerThrottlingEnabled()
conditionally based on ENABLE(HIDDEN_PAGE_DOM_TIMER_THROTTLING).

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@146729 268f45cc-cd09-0410-ab3c-d52691b4dbfc