Stop tracking a drag after the user drops the dragged item.

BUG=44407
TEST=Perform the steps outlined in the bug and verify that after step #5 the folders are no longer still active.

Review URL: http://codereview.chromium.org/2849026

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@51132 0039d316-1c4b-4281-b951-d872f2087c98