[Mac] Give every Clipboard an underlying Pasteboard
https://bugs.webkit.org/show_bug.cgi?id=115979

Reviewed by Andreas Kling.

This is the first step in Clipboard and Pasteboard refactoring
to fix the overlap and many layering violations.

* dom/Clipboard.cpp:
(WebCore::Clipboard::Clipboard): Add an m_pasteboard data member,
Mac-only for now, but eventually for all platforms.
(WebCore::Clipboard::~Clipboard): No longer inline the destructor.

* dom/Clipboard.h: Add WTF_USE_LEGACY_STYLE_ABSTRACT_CLIPBOARD_CLASS
flag, currently true for all non-Mac platforms. Make the constructor
no longer inline. Added a PassOwnPtr<Pasteboard> argument to the
constructor and an OwnPtr<Pasteboard> data member.

* platform/Pasteboard.h: Added a create function to make it easy
to create a pasteboard given a pasteboard name.

* platform/mac/ClipboardMac.mm:
(WebCore::ClipboardMac::ClipboardMac): Create a pasteboard and pass
it to the Clipboard constructor.

* platform/mac/PasteboardMac.mm:
(WebCore::Pasteboard::create): Added.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149972 268f45cc-cd09-0410-ab3c-d52691b4dbfc