user_activity: Fix regression in the benchmark_metrics_expriment.

The child fractions were not computed correctly after I updated the
extracted fields from the data collection scripts.

BUG=None
TEST=None

Change-Id: I8e672e0c245696e3351f1be9a3b391f46accea9f
Reviewed-on: https://chrome-internal-review.googlesource.com/298619


Commit-Queue: Evelina Dumitrescu <evelinad@google.com>
Tested-by: Evelina Dumitrescu <evelinad@google.com>
Reviewed-by: Luis Lozano <llozano@chromium.org>
Reviewed-by: Evelina Dumitrescu <evelinad@google.com>
Reviewed-by: George Burgess <gbiv@google.com>
Reviewed-on: https://chromium-review.googlesource.com/435918


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