Remove getCurrentMountPointUrl from the directory model.

This function uses paths and it is redundant, therefore it is removed by this patch.

TEST=Tested manually.
BUG=328252

Review URL: https://codereview.chromium.org/98253011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243072 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243072