[GTK] REGRESSION(r171742): Test /webkit2/WebKitWebView/disallow-modal-dialogs fails
https://bugs.webkit.org/show_bug.cgi?id=135412

Reviewed by Philippe Normand.

The problem is that WebPageProxy::setUIClient() calls
setCanRunModal() with the value of m_uiClient->canRunModal() which
is always true for us. We override that manually by calling
setCanRunModal() depending on the WebKitSettings. In r171742,
webkitWebViewUpdateSettings(), the method that overrides
setCanRunModal(), was moved before attachUIClientToView(), so the
value set by WebPageProxy::setUIClient() is the last one and takes
precedence. We need to make sure webkitWebViewUpdateSettings() is
always called after attachUIClientToView().

* UIProcess/API/gtk/WebKitWebView.cpp:
(webkitWebViewConstructed): Move webkitWebViewUpdateSettings()
call after attachUIClientToView() and add a comment to not forget
about it again.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@171796 268f45cc-cd09-0410-ab3c-d52691b4dbfc