Get rid of of custom code for Clipboard.setDragImage()

Introduce a setDragImage() method in Clipboard whose prototype matches the
JavaScript one. Have this method call setDragImageResource() or
setDragImageElement() depending on the input type.

The first parameter type in Web IDL is now an Element, to match the
specification:
http://www.w3.org/html/wg/drafts/html/master/editing.html#datatransfer

There is no web-exposed behavior change.

TEST=ManualTests/drag-with-div-or-image-as-data-image.html
R=haraken

Review URL: https://codereview.chromium.org/26532002

git-svn-id: svn://svn.chromium.org/blink/trunk@159196 bbb929c8-8fbe-4397-9dbb-9b2b20218538