[Mac] Thin out the ClipboardMac class and header file to prepare for deleting them
https://bugs.webkit.org/show_bug.cgi?id=116159

Reviewed by Andreas Kling.

Source/WebCore:

* dom/Clipboard.h:
(WebCore::Clipboard::pasteboard): Added. For clients that have a DOM clipboard
object and need to get to the Pasteboard platform abstraction.

* editing/Editor.cpp:
(WebCore::Editor::dispatchCPPEvent): Call writePasteboard instead of writeClipboard.
Our long term plan is to delete writeClipboard.

* platform/Pasteboard.h: Added writePasteboard function.
(WebCore::Pasteboard::name): Made this const.

* platform/mac/ClipboardMac.h: Deleted unneeded includes and forward declarations.
Deleted all the data members and the virtual destructor.

* platform/mac/ClipboardMac.mm: Deleted the many includes that are no longer needed.
Deleted the destructor, since it's now automatically generated. Deleted the comment
on an #endif since it's only a few source lines away from the #if.
(WebCore::ClipboardMac::ClipboardMac): Deleted the code to set the data members,
since they are no longer used.

* platform/mac/PasteboardMac.mm: Changed include to Clipboard.h instead of ClipboardMac.h.
(WebCore::Pasteboard::writePasteboard): Replaced writeClipboard with this.

Source/WebKit/mac:

* WebCoreSupport/WebDragClient.mm:
(WebDragClient::willPerformDragSourceAction): Changed to get the pasteboard name
without going through the ClipboardMac class.
(WebDragClient::startDrag): Ditto.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@150122 268f45cc-cd09-0410-ab3c-d52691b4dbfc