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Previously, active sessions and recent users were reported separately,
that is confusing on the server side.
This change adds 'user_email' field to ActiveTimePeriod proto message
and populates it for affiliated users that are eligible for reporting.
Also, user email is reported only if user reporting is turned on.

Initially landed in https://crrev.com/c/904527
Reverted in https://crrev.com/c/923228

TBR=xiyuan@chromium.org

BUG=810093
TEST=Browser tests added.
TEST=ASAN_OPTIONS="detect_leaks=1" ./out/asan/browser_tests --gtest_filter=DeviceStatusCollectorTest*
TEST=ASAN_OPTIONS="detect_leaks=1" ./out/asan/browser_tests --gtest_filter=Blub*

Change-Id: I9db2c07f6aa0ba89b9f5fc6fc890207ad5e9a97b
Reviewed-on: https://chromium-review.googlesource.com/924005
Commit-Queue: Sergey Poromov <poromov@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537641}