Complete the plug-in URL string before sending it to the UI process
https://bugs.webkit.org/show_bug.cgi?id=111355
<rdar://problem/13326713>

Reviewed by Andreas Kling.

If the URL is not absolute, we won't be able to find it by path extension in the plug-in info store.

* WebProcess/WebCoreSupport/WebChromeClient.cpp:
(WebKit::WebChromeClient::unavailablePluginButtonClicked):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@144659 268f45cc-cd09-0410-ab3c-d52691b4dbfc