[ash] Enable dragging an item from overflow to shelf

This CL implements
a) rip off from overflow, drag into shelf and drops the item.
b) rip off from overflow, drag into shelf and rip off again from shelf.
  (Same state with ripped off from overflow)

R=jamescook@chromium.org, skuhne@chromium.org
BUG=137141
TEST=ShelfViewTest.CheckDragAndDropFromOverflowBubbleToShelf

Review URL: https://codereview.chromium.org/107023004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239950 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239950