Do not migrate proxied connections on degradation or write error

When either of these conditions occur, if the session is proxied, do
nothing. The outer session (direct to the proxy server) may still
migrate, but the proxied connection cannot.

Because sessions carrying tunnels use a bigger max packet size, this
required some changes to the test packet creator.

Bug: 338342162
Change-Id: Ie9c70409b810a38c69c71ee73544ca4792ae5b90
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5593811


Reviewed-by: Ryan Hamilton <rch@chromium.org>
Commit-Queue: Dustin Mitchell <djmitche@chromium.org>
Reviewed-by: Fan Yang <fayang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312196}