Sometimes WKWebView is blank after resuming the app, until you scroll

Sometimes WKWebView is blank after resuming the app, until you scroll
https://bugs.webkit.org/show_bug.cgi?id=135275
<rdar://problem/17803170>

Reviewed by Benjamin Poulain.

* UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::dispatchViewStateChange):
If the UI process is waiting for a didUpdateViewState, we need to *always*
get a reply from the Web Process, so dispatchViewStateChange should *always*
send SetViewState even if nothing changed (so that we get the reply).

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@171570 268f45cc-cd09-0410-ab3c-d52691b4dbfc