Preparation for <http://webkit.org/b/31414> Implement new SPI for
dealing with user scripts/stylesheets and isolated worlds

Reviewed by Dave Hyatt.

* WebView.cpp:
(toString):
(toKURL):
Added these helper functions to convert BSTRs to WebCore types.

(toStringVector):
(WebView::addUserScriptToGroup):
(WebView::addUserStyleSheetToGroup):
(WebView::removeUserScriptFromGroup):
(WebView::removeUserStyleSheetFromGroup):
(WebView::removeUserScriptsFromGroup):
(WebView::removeUserStyleSheetsFromGroup):
(WebView::removeAllUserContentFromGroup):
Use the new helper functions.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@50906 268f45cc-cd09-0410-ab3c-d52691b4dbfc