Extract embedded test server logic into a mixin

Bug: 952902
Change-Id: I199cbd0b6f046f388f6e27a2031e5544a015a316
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1574647
Commit-Queue: Yulun Wu <yulunwu@chromium.org>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653430}
7 files changed