Fixes bugs in packet size computation in the creator and in the framer

Fixes bugs in packet size computation in the creator and in the framer
mostly related to truncated ack size computation and verification. Fixes
a bug where sequence_number_length was incorrectly being allowed to
change mid-packet, and actively updates lengths in packets at
packet/FEC-group boundary. Adds tests to verify correct detection of ack
truncation.

Much yak hair was harvested, but no yaks were hurt in the making of
this CL.

Fixes bugs in packet-size computation in QuicPacketCreator and adds
tests.

Merge internal change: 67880898

R=rch@chromium.org

[Ryan Hamilton, 2014-05-28 00:29:56]

lgtm