Fix handling dragging onto the volume list.

Previously, when dragging out of the item, the cursor wasn't changing 'dragging not allowed' because of a JS error.

TEST=Tested manually.
BUG=338636

Review URL: https://codereview.chromium.org/145773005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248995 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248995