user_activity: Fixed a regression in ComputeScoreForBenchmarkSet.

The score fraction for a group was computed wrong.
The fraction should be computed as the score of the group over the
score of that group in the ideal case (each CWP function matching that group
should have the 1.0 value).

BUG=None
TEST=None

Change-Id: I294b42f0c8112ab9bd6f5d983876b6d779aad5e4
Reviewed-on: https://chrome-internal-review.googlesource.com/297675


Tested-by: Evelina Dumitrescu <evelinad@google.com>
Reviewed-by: Evelina Dumitrescu <evelinad@google.com>
Reviewed-by: Luis Lozano <llozano@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/435921


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