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This CL addresses a bug where engagement cleanup is continually run in
incognito mode because we do not write a pref value for the last
engagement. When in incognito, engagement is never regarded as going
stale, meaning that cleanup is never run. A test is added to ensure the
correct behaviour.

The frequency of calls to Clock::Now() is also reduced in this CL.

BUG=947835

Change-Id: I7c91caa98a3c7e7c6e39aec314682f382bb2581b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1895097
Auto-Submit: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Commit-Queue: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712035}