Make Pasteboard deal with ChromiumDataObject instead of Clipboard.

ChromiumDataObject is a platform class; Clipboard is a DOM class.
This works towards removing the layering violation caused by Pasteboard
knowing too much about the DOM.

BUG=276565

Review URL: https://chromiumcodereview.appspot.com/22929033

git-svn-id: svn://svn.chromium.org/blink/trunk@157242 bbb929c8-8fbe-4397-9dbb-9b2b20218538