Fix nits in feature_engagement::SessionDurationUpdater.

- Mark methods const.
- Put includes in the right places.
- Remove unneeded explicit / virtual keywords.
- Declare a variable where used.
- Add comments were needed; Remove useless comments.

Change-Id: Ie342d8712f9a7e7a542401352371f7d348555e6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1525622
Commit-Queue: Collin Baker <collinbaker@chromium.org>
Reviewed-by: Collin Baker <collinbaker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641229}
2 files changed