[SyncFS] Handle remote app-root deletion

BUG=240165
TEST=manual

Review URL: https://codereview.chromium.org/109983002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239497 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239497