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There's a unittest that reaches into FidoCableDevice's internals, and
thus needs to be a friend class, in order to check that the sequence
numbers are being incremented. But the prior test already confirms that
the encryption and decrypt is working as expected by checking inputs and
outputs, and testing the behaviour is to be preferred to testing the
internals.

Therefore, delete that superfluous test and eliminate one of the
friends.

Another test needs to set the sequence counters. Better to add a testing
interface for that than to expose all private members. Thus another
friend can be dropped.

Then |FidoCableDevice::EncryptionData| can be made private.

Change-Id: I42ea49b14b4a4b4e0afa2a4e5490fa9075439fe8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1776796
Commit-Queue: Adam Langley <agl@chromium.org>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#692301}