[Qt] UIProcess should initialize WebCoreSystemInterface.
https://bugs.webkit.org/show_bug.cgi?id=113300

The WebCoreSystemInterface is necessary to detect supported
mime types on Mac OS. In particular it is needed for download
support.
This patch fixes in particular the previously failing
Qt API qmltest WebViewDownload::test_downloadRequest.
The test was crashing when using QTKit, because functions
in WebCoreSystemInterface could not be resolved on the UIProcess side.

Reviewed by Jocelyn Turcotte.

* UIProcess/qt/WebContextQt.cpp:
(WebKit::WebContext::platformInitializeWebProcess):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@146880 268f45cc-cd09-0410-ab3c-d52691b4dbfc