[ash] Move LauncherItem to the front of first panel item during the dragging off from the shelf

When IconProxy is dragging, it affects panel item position if launcher item is moved
to the last position of LauncherModel.
To handle this, launcher item should be moved to the front of first panel item
during the IconProxy dragging.

R=skuhne@chromium.org
BUG=305120
TEST=visual test

Review URL: https://codereview.chromium.org/26165004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227587 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227587