Build fix for smart compliers.

Reviewed by Tim Horton.

[NSDictionary dictionaryWithObjectsAndKeys:] returns an NSDictionary,
not a NSMutableDictionary.

* WebCoreSupport/WebEditorClient.mm:
(WebEditorClient::updateSpellingUIWithGrammarString):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149149 268f45cc-cd09-0410-ab3c-d52691b4dbfc