user_activity: Added unit tests for benchmark metrics experiment.

BUG=None
TEST=None

Change-Id: I8876f7ad4d4f05dad7e8329984f972a94df58edf
Reviewed-on: https://chrome-internal-review.googlesource.com/287961


Tested-by: Evelina Dumitrescu <evelinad@google.com>
Reviewed-by: George Burgess <gbiv@google.com>
Reviewed-by: Ting-Yuan Huang <laszio@google.com>
Reviewed-by: Evelina Dumitrescu <evelinad@google.com>
Reviewed-on: https://chromium-review.googlesource.com/435907


Commit-Ready: Luis Lozano <llozano@chromium.org>
Tested-by: Luis Lozano <llozano@chromium.org>
Reviewed-by: Luis Lozano <llozano@chromium.org>