Add Finch experiment for deleting session cookies at shutdown.
Session cookies get deleted at startup. If at least one cookie gets deleted,
there is a non-trivial impact on startup time. The goal is to move this I/O
intensive task from startup to shutdown, and measure the impact on startup and
shutdown. Session cookies still get deleted at startup, but this operation is
expected to be fast if no cookies are deleted.
BUG=488314
Review URL: https://codereview.chromium.org/
1128233009
Cr-Commit-Position: refs/heads/master@{#330992}