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This is important for desktop, as >1% of events emitted are hitting the
max.  By my estimates from looking at the distribution, it's probably
safe to change the max to more like 6-10 hours to have fewer than 1% of
emits on desktop in the overflow bucket.  I decided to use one day
as the max simply because it will make it easier to identify possible
issues in the future with this metric.  (E.g., we should not see
two emits to the overflow bucket on a single day, otherwise something
is wrong.)

While at it, revise Session.TotalDuration description to explain how
it changed in M68.

BUG=856467

Change-Id: If0fcead52a15af24899618448b9c72bb87fb3e2b
Reviewed-on: https://chromium-review.googlesource.com/1147210Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577611}