Rename Clipboard.idl to DataTransfer.idl to match the spec

http://www.whatwg.org/specs/web-apps/current-work/multipage/dnd.html#the-datatransfer-interface

The spec uses DataTransfer and both Firefox and IE agrees.

BUG=348580
R=ch.dumez@samsung.com, tkent@chromium.org

Review URL: https://codereview.chromium.org/183743013

git-svn-id: svn://svn.chromium.org/blink/trunk@168494 bbb929c8-8fbe-4397-9dbb-9b2b20218538