[Mac] Change Clipboard::create functions so they don't use ClipboardMac::create any more
https://bugs.webkit.org/show_bug.cgi?id=116163

Reviewed by Anders Carlsson.

* dom/Clipboard.h: Made the file drag boolean default to false, better for most callers.

* platform/mac/ClipboardMac.mm: Remove #if ENABLE(DRAG_SUPPORT); that's always on for Mac
and it's not helpful to have the conditionals in here. It's helpful in cross-platform
source files.
(WebCore::Clipboard::create): Create a Pasteboard and then create a Clipboard with new.
Later could refactor to have the Pasteboard creation be platform-specific, and make these
functions platform-independent.
(WebCore::Clipboard::createForDragAndDrop): Ditto.
(WebCore::Clipboard::createForCopyAndPaste): Ditto.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@150127 268f45cc-cd09-0410-ab3c-d52691b4dbfc