[Mac] Make Clipboard class no longer polymorphic by removing the last virtual functions

Reviewed by Anders Carlsson.

* WebCore.exp.in: Add ~Clipboard since it's no longer a virtual function, but to the
non-iOS section, since iOS is still using the legacy version of Clipboard.

* dom/Clipboard.h: Mark items non-virtual, too. It's not implemented on Mac yet.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@150138 268f45cc-cd09-0410-ab3c-d52691b4dbfc
3 files changed