[Mac] Move clearAllData from ClipboardMac to PasteboardMac
https://bugs.webkit.org/show_bug.cgi?id=116001

Reviewed by Andreas Kling.

* dom/Clipboard.cpp:
(WebCore::Clipboard::clearAllData): Added. Calls through to Pasteboard after
doing the canWriteData check, which is part of DOM rules, not the platform.

* dom/Clipboard.h: Made clearAllData non-virtual in non-legacy case.

* platform/mac/ClipboardMac.h: Removed clearAllData.
* platform/mac/ClipboardMac.mm: Ditto.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149979 268f45cc-cd09-0410-ab3c-d52691b4dbfc