Do not allow ephemeral apps to retain filesystem entries

After an ephemeral app is closed, all retained file and
directory entries are removed.

BUG=348866
TEST=browser_tests
(EphemeralAppBrowserTest.DisableRetainFileSystemEntries,
FileSystemApiTest.*)

Review URL: https://codereview.chromium.org/177443003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255019 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255019