Make sure the sync directory is deleted on sign-out, take 2.
Previously, if the user had turned off sync before signing out, the
directory would not be deleted correctly.
A similar CL was submitted in http://crrev.com/
682173002 and
subsequently reverted in http://crrev.com/
708153004 due to two
different crashes.
This CL simplifies things by doing everything on the sync thread, which
it turns out is still alive when we need it.
BUG=424593, 431421, 431354
Review URL: https://codereview.chromium.org/
742453002
Cr-Commit-Position: refs/heads/master@{#304856}