user_activity_benchmarks: Added unit tests for the process_hot_functions

module.

BUG=None
TEST=None

Change-Id: I41fb0cd89e108f96cc061bf3ac72403a7108b30b
Reviewed-on: https://chrome-internal-review.googlesource.com/279955


Commit-Queue: Evelina Dumitrescu <evelinad@google.com>
Tested-by: Evelina Dumitrescu <evelinad@google.com>
Reviewed-by: George Burgess <gbiv@google.com>
Reviewed-by: Luis Lozano <llozano@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/435456


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