Flush shared memory when modified.

(Original patch by Darren Krahn <dkrahn@chromium.org>)

This change keeps .stmapfile in sync and eliminates the need to delete
it every session.

BUG=chromium-os:18869, chromium-os:19807
TEST=Executed the following manual tests:
- Configured Google-A certificate and verified that it was retained
  across logins and restarts (with .stmapfile deletion disabled).
- Imported a certificate and verified that is was retained across logins
  and restarts (with .stmapfile deletion disabled).

Change-Id: I547a3208cb48d904fde6bf389979955ca7fb85ea
6 files changed