user_activity: Added script to select the optimal benchmark set.

It generates all possible combinations of benchmark sets of a given size
and it computes for every set a metric. Three different metrics have been
used: function count, distance variation and score.

TEST=None
BUG=None

Change-Id: I4e4af9f46a3056cfbcdc9d89b2c1333650b41ccc
Reviewed-on: https://chrome-internal-review.googlesource.com/292435


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/435914


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