dispatchViewStateChange should not wait for sync reply if the page isn't visible

dispatchViewStateChange should not wait for sync reply if the page isn't visible
https://bugs.webkit.org/show_bug.cgi?id=145242
<rdar://problem/20967937>

Reviewed by Ben Poulain.

This is particularly problematic on iOS, since if the page isn't visible the process is likely suspended.

* UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::dispatchViewStateChange):
(WebKit::WebPageProxy::waitForDidUpdateViewState):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@184692 268f45cc-cd09-0410-ab3c-d52691b4dbfc