Assertion in LegacyWebArchive::create() in editing tests
https://bugs.webkit.org/show_bug.cgi?id=112642

Reviewed by Simon Fraser.

Address Simon's review comment. Don't convert String to MIMEType just to convert it back to String.

* platform/mac/PasteboardMac.mm:
(WebCore::fragmentFromWebArchive):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@146391 268f45cc-cd09-0410-ab3c-d52691b4dbfc