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In particular, migrate SocketsUdpApiTest.SocketsUdpExtension
and SocketApiTest.SocketUDPExtension.

This CL introduces a new C++ TestUdpEchoServer class, which echoes
back the data it receives over an ephemeral UDP socket, and makes
those two tests use it.

They had been the only consumers of the SpawnedTestServer's UDP_ECHO
mode, so once this lands, that mode can be removed from the test
server.

The reason for migrating tests off of SpawnedTestServer is that
it has been a source of flaky failures and timeouts for years,
so everything that can use an in-process C++ server should be
doing so.

Bug: 492672
Change-Id: Iec532fc94ee9c33c612fca3d9392388b594707e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2513787Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823307}