user_activity: Added implementation for the metrics experiment.

The validation of the benchmark metrics is done on a pair of CWP
function sets.

BUG=None
TEST=benchmark_metrics_validation_unittest passes

Change-Id: I15cb5976ba4d9a41ff97376b0ac3320e9a798fd5
Reviewed-on: https://chrome-internal-review.googlesource.com/286160


Reviewed-by: Luis Lozano <llozano@chromium.org>
Tested-by: Evelina Dumitrescu <evelinad@google.com>
Reviewed-on: https://chromium-review.googlesource.com/435911


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