REGRESSION: WebProcess is terminated when all Safari windows are closed.
<rdar://problem/13990901>
<http://webkit.org/b/116766>

Reviewed by Alexey Proskuryakov.

When closing the last page, only disconnect the web process if we're using a network process.
Otherwise there might be session state in the web process getting lost.

* UIProcess/WebProcessProxy.cpp:
(WebKit::WebProcessProxy::removeWebPage):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@150691 268f45cc-cd09-0410-ab3c-d52691b4dbfc