Add CHECKs in QuicChromiumPacketWriter

To investigate the linked bug.

Bug: 1376754
Change-Id: Ia39276a129f4a189f1e0e1db964b3c493f57b717
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4551585


Reviewed-by: Ryan Hamilton <rch@chromium.org>
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1147542}