Split TrafficPolicer into a generic one-way packet filter class and a policer-specific logic.

Split TrafficPolicer into a generic one-way packet filter class and a policer-specific logic.

The intent here is to be able to create a filter that loses packets based on
other patterns (random loss, cyclic loss, etc) without having to write the
port-related boilerplate again.

n/a (test-only change)

Merge internal change: 139945285

BUG=

[Ryan Hamilton, 2016-12-01 21:15:14]

rch@chromium.org changed reviewers:
+ zhongyi@chromium.org

[Zhongyi Shi, 2016-12-01 22:29:30]

lgtm