user_activity: Moved some functions to the utils module.

Moved ComputeCWPCummulativeInclusiveStatistics and ComputeCWPChildFunctionsFractions
from the process_hot_functions module to the utils module.

BUG=None
TEST=None

Change-Id: Ib5968ae722cc8a775744418f24512e3d727d3936
Reviewed-on: https://chrome-internal-review.googlesource.com/298618


Commit-Queue: Evelina Dumitrescu <evelinad@google.com>
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/435919


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