[GTK] Add API to prefetch DNS of a given hostname to WebKit2 GTK+ API
https://bugs.webkit.org/show_bug.cgi?id=99695

Reviewed by Anders Carlsson.

* UIProcess/API/gtk/WebKitWebContext.cpp:
(webkit_web_context_prefetch_dns): Public method to resolve the
domain name in advance for the given hostname.
* UIProcess/API/gtk/WebKitWebContext.h:
* UIProcess/API/gtk/docs/webkit2gtk-sections.txt: Add
webkit_web_context_prefetch_dns.
* WebProcess/InjectedBundle/API/gtk/WebKitWebExtension.cpp:
(webkitWebExtensionDidReceiveMessage): Parse PrefetchDNS message
and call WebCore::prefetchDNS() with the given hostname.
(didReceiveMessage): Call webkitWebExtensionDidReceiveMessage().
(webkitWebExtensionCreate): Add implementation for
didReceiveMessage callback.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@141102 268f45cc-cd09-0410-ab3c-d52691b4dbfc