Move isReadOnly to EntryLocation.

This CL moves the isReadOnly method from the directory model to the
EntryLocation, which can be created via the volume manager.

Along the way the code has been cleaned up. The isDriveOffline has been removed
from the file manager, since it is accessible from the volume manager. This
reduces dependency spaghetti.

TEST=Tested manually. Partly covered by browser_tests.
BUG=328260

Review URL: https://codereview.chromium.org/107963004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242583 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242583