relnote: Non-functional change. Cleanup pg3 support for quic_packet_reader

relnote: Non-functional change. Cleanup pg3 support for quic_packet_reader

Bring it more inline with Chromium source but cleanup the definition of
MMSG_MORE macro to exist in the header so that there's no violation of
ODR for QUicPacketReader class based upon inclusion.

Merge internal change: 141357226

BUG=

[Zhongyi Shi, 2016-12-13 02:39:09]

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

[Ryan Hamilton, 2016-12-13 03:51:45]

lgtm