[Mac] Make Clipboard::create functions for Mac platform independent by moving Pasteboard creation to Pasteboard functions
https://bugs.webkit.org/show_bug.cgi?id=116179

Reviewed by Andreas Kling.

* dom/Clipboard.cpp:
(WebCore::Clipboard::createForCopyAndPaste): Moved the function here from
ClipboardMac, and have it use Pasteboard functions that are not Mac-specific.
(WebCore::Clipboard::create): Ditto.
(WebCore::Clipboard::createForDragAndDrop): Ditto.

* platform/DragData.h:
(WebCore::DragData::pasteboardName): Made this const.

* platform/Pasteboard.h: Added new create functions, used by the clipboard
create functions above.

* platform/mac/ClipboardMac.mm: Removed the functions that were moved to the
Clipboard.cpp file.

* platform/mac/PasteboardMac.mm:
(WebCore::Pasteboard::createForCopyAndPaste): Added.
(WebCore::Pasteboard::createPrivate): Added.
(WebCore::Pasteboard::createForDragAndDrop): Added.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@150183 268f45cc-cd09-0410-ab3c-d52691b4dbfc