user_activity: Integrated benchmark_metrics_experiment with utils.

Removed duplicate code from the process_hot_functions and
benchmark_metrics_experiment.

BUG=None
TEST=benchmark_metrics_experiment passes

Change-Id: I11ed11b4f1cbb573e1d10a2795570b4820602031
Reviewed-on: https://chrome-internal-review.googlesource.com/291202


Reviewed-by: George Burgess <gbiv@google.com>
Reviewed-by: Ting-Yuan Huang <laszio@google.com>
Reviewed-by: Evelina Dumitrescu <evelinad@google.com>
Tested-by: Evelina Dumitrescu <evelinad@google.com>
Reviewed-on: https://chromium-review.googlesource.com/435912


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