Lower down the priority, as we think PacketReadError after handshake is low.  

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6bdb765e4834447e7916d0b0d93670d36a6c8970

commit 6bdb765e4834447e7916d0b0d93670d36a6c8970
Author: Zhongyi Shi <zhongyi@chromium.org>
Date: Sat Mar 10 02:24:57 2018

Do not close connection when receiving read errors during pending
migration. Also add more granular histograms to collect data on
PacketReader errors.

Bug: 818050
Change-Id: I39d736967f1a3347e964c09862982941e4f0d60d
Reviewed-on: https://chromium-review.googlesource.com/956663
Commit-Queue: Zhongyi Shi <zhongyi@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Ryan Hamilton <rch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542322}
[modify] https://crrev.com/6bdb765e4834447e7916d0b0d93670d36a6c8970/net/quic/chromium/quic_chromium_client_session.cc
[modify] https://crrev.com/6bdb765e4834447e7916d0b0d93670d36a6c8970/tools/metrics/histograms/histograms.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f3d6cddb68a67df9255b0384286cd3a1c3a8004d

commit f3d6cddb68a67df9255b0384286cd3a1c3a8004d
Author: Zhongyi Shi <zhongyi@chromium.org>
Date: Wed Jul 11 03:30:02 2018

Quic Connection Migration: remove |migration_pending_|.

Currently |migration_pending_| is mixing three invariants together:
- connection migration is triggered but no alternate network is available for migration.
- connection migration on write error posted, but other direct migration completes, prevent the posted task from being executed.
- connection migration on write error is in progress, read error should be ignored.

This change explicitly separate out those three invariants:
- |wait_for_new_network_| tracks if a migration is triggered and no alternate network is available
- MigrateSessionOnWriteError(posted task to handle write error) takes the writer which encountered the write error, and compares with the active writer in use to determine if the migration should be aborted.
- |ignore_read_error_| tracks if read error should be ignored.

Bug: 818050
Change-Id: Ic6d1c4c95f9da2207901ba985fb662ab0d4abc3c
Reviewed-on: https://chromium-review.googlesource.com/1129549
Commit-Queue: Zhongyi Shi <zhongyi@chromium.org>
Reviewed-by: Ryan Hamilton <rch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574056}
[modify] https://crrev.com/f3d6cddb68a67df9255b0384286cd3a1c3a8004d/net/quic/chromium/quic_chromium_client_session.cc
[modify] https://crrev.com/f3d6cddb68a67df9255b0384286cd3a1c3a8004d/net/quic/chromium/quic_chromium_client_session.h
[modify] https://crrev.com/f3d6cddb68a67df9255b0384286cd3a1c3a8004d/net/quic/chromium/quic_chromium_packet_writer.cc
[modify] https://crrev.com/f3d6cddb68a67df9255b0384286cd3a1c3a8004d/net/quic/chromium/quic_stream_factory_test.cc