[Mac] Move hasData from ClipboardMac to PasteboardMac
https://bugs.webkit.org/show_bug.cgi?id=115999

Reviewed by Andreas Kling.

* dom/Clipboard.cpp:
(WebCore::Clipboard::hasData): Added. Calls through to Pasteboard.

* dom/Clipboard.h: Added LEGACY_VIRTUAL and LEGACY_PURE macros,
only for within this header file. These help us keep many functions
as pure virtual in the legacy style Clipboard class, but have them
be non-virtual in the normal/future style. Use LEGACY_VIRTUAL and
LEGACY_PURE to make the hasData member non-virtual.

* platform/Pasteboard.h: Added hasData, and added some blank lines and a FIXME.

* platform/mac/ClipboardMac.h: Removed hasData override.
* platform/mac/ClipboardMac.mm: Ditto.

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

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149976 268f45cc-cd09-0410-ab3c-d52691b4dbfc