Change the null value for packet sequence numbers to

Change the null value for packet sequence numbers to
kInvalidPacketSequenceNumber (== 0), which cannot appear as a packet
sequence number.

Update QuicFecGroupTest::UpdateAndRevive and
QuicFecGroupTest::UpdateAndReviveOutOfOrder to generate packet numbers
starting with 1 rather than 0.

Change the null value for packet sequence numbers to
kNullPacketSequenceNumber (== 0).

Merge internal change: 87819844

R=rch@chromium.org

[Ryan Hamilton, 2015-03-07 03:17:49]

lgtm