[Mac] Move setDragImage from ClipboardMac to PasteboardMac
https://bugs.webkit.org/show_bug.cgi?id=116036

Reviewed by Anders Carlsson.

* dom/Clipboard.cpp:
(WebCore::Clipboard::~Clipboard): Stop loading the drag image when
the clipboard object is deallocated. It would be better if there was
a more-well-defined time to stop the load.
(WebCore::Clipboard::setDragImage): Added. Sets up a drag image
based on a CachedImage. This tells the image loader to load the image
and updates the drag image when the image arrives. Copied from the
code in ClipboardMac, but refactored to use a separate loader object.
(WebCore::Clipboard::setDragImageElement): Added. Sets up a drag image
based on an element.
(WebCore::Clipboard::updateDragImage): Added. Common code to update
drag image when a new one is set or when a drag image load completes.
(WebCore::DragImageLoader::create): Added.
(WebCore::DragImageLoader::DragImageLoader): Ditto.
(WebCore::DragImageLoader::startLoading): Ditto.
(WebCore::DragImageLoader::stopLoading): Ditto.
(WebCore::DragImageLoader::imageChanged): Ditto.

* dom/Clipboard.h: Made setDragImage and setDragImageElement non-virtual
for non-legacy. Added updateDragImage and m_dragImageLoader.

* platform/Pasteboard.h: Added setDragImage function.

* platform/mac/ClipboardMac.h: Removed CachedImageClient as a base class
for ClipboardMac. Removed setDragImage and setDragImageElement.

* platform/mac/ClipboardMac.mm: Removed setDragImage and setDragImageElement.
(WebCore::ClipboardMac::~ClipboardMac): Removed code to remove self as
client loading the drag image. This is now in the Clipboard base class.

* platform/mac/PasteboardMac.mm:
(WebCore::Pasteboard::setDragImage): Added. Contains code moved here
from ClipboardMac.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@150100 268f45cc-cd09-0410-ab3c-d52691b4dbfc