Landing Recent QUIC Changes.

Landing Recent QUIC Changes.

Simplify QuicUnackedPacketMap's RemovePreviousTransmissions by always
removing all old transmissions.

When truncated acks start arriving, they don't stop until least_unacked
is raised, which takes an entire RTT, so this is the eventual effect of
all those truncated acks.  Instead, remove all of them the first time.

Merge internal change: 74881713
https://codereview.chromium.org/556103002/

Discard old packets from the QuicSentPacketManager which are only
present for RTT purposes(typically acks) when the map exceeds 200 packets.

Merge internal change: 74814618
https://codereview.chromium.org/551223002/

Fix a flaky QUIC end_to_end_test when the socket was suddenly unwritable
and the connection closed, but CanWrite returned true.

Merge internal change: 74798317
https://codereview.chromium.org/550703003/

Fix a crash bug in QuicUnackedPacketMap when truncated acks arrived.

Merge internal change: 74792668
https://codereview.chromium.org/554993002/

Send an ack when it 20 packets have been received since the last ack was
sent.

This allows the peer to discard data from the SentPacketManager.

Merge internal change: 74741188
https://codereview.chromium.org/555653002/

R=rch@chromium.org

Review URL: https://codereview.chromium.org/561513003

Cr-Commit-Position: refs/heads/master@{#294295}