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Took me too long to figure out why all the tests were dropping three
bytes from the beginning of messages. Therefore it's worth a named
constant.

Change-Id: I3b22d9c4e654dd67b6de9ef2cb3cbe4afb358c61
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1776795
Commit-Queue: Adam Langley <agl@chromium.org>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#692262}