Check that proxied sessions do not migrate

Having addressed all of the triggers for migration, this check should
never fail.

Bug: 338342162
Change-Id: Icf5f61a664ce5a4a5527c63b666fc3ef8c941de3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5605288


Reviewed-by: Fan Yang <fayang@chromium.org>
Commit-Queue: Dustin Mitchell <djmitche@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312197}