Limit usage of mount paths in Files app.

This is an initial cleanup aiming at completely removal of mount paths from Files app.

TEST=Tested manually by mounting and unmounting archives.
BUG=316077

Review URL: https://codereview.chromium.org/149263004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248118 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248118