Add coverage pramas to stats_test.py to indicate uncovered lines.
Add coverage pramas to stats_test.py to indicate uncovered lines.

Somehow evaded the CQ trybot in https://codereview.chromium.org/467263005?

TBR=alancutter@chromium.org, eseidel@chromium.org
BUG=

Review URL: https://codereview.chromium.org/495113002

nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra