Convert DirectoryModel.getCurrentRootType to use volume manager.

The root type can be acquired from the LocationInfo, so we don't need to use fullPaths anymore for it.

TEST=Tested manually.
BUG=328252

Review URL: https://codereview.chromium.org/121193002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242529 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242529