Reject filesystem root entries as args for chrome.fileSystem functions.

BUG=333600

Review URL: https://codereview.chromium.org/139543005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245468 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245468