[Mac] Move clearData from ClipboardMac to PasteboardMac
https://bugs.webkit.org/show_bug.cgi?id=116000

Reviewed by Andreas Kling.

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

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

* platform/Pasteboard.h: Added an overload of the clear function that takes
a single type and clears only that type. Also temporarily exported the
cocoaTypeFromHTMLClipboardType function as a static member so it can be used
in both ClipboardMac and PasteboardMac during the transition.

* platform/mac/ClipboardMac.h: Removed clearData.
* platform/mac/ClipboardMac.mm: Removed clearData.
(WebCore::ClipboardMac::getData): Changed to call cocoaTypeFromHTMLClipboardType
as a Pasteboard class static member function.
(WebCore::ClipboardMac::setData): Ditto.

* platform/mac/PasteboardMac.mm:
(WebCore::Pasteboard::cocoaTypeFromHTMLClipboardType): Moved here from
ClipboardMac.
(WebCore::Pasteboard::clear): Added. Moved here from ClipboardMac.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149977 268f45cc-cd09-0410-ab3c-d52691b4dbfc