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This is sort of a draft but it may not get refined more than this. The
code itself is commented, this is intended more as orientation so
someone trying to use it can figure out what they're looking at enough
that the comments are of any use. ScopedXPCServiceMock interposes a
mock that is then _implicitly_ picked up by the code under test, so
directly tracing it is not likely to be the best place to start...

Change-Id: Ia851b3005788b3200a08ae572ee48fc29f822f47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2491222
Commit-Queue: Adam Norberg <norberg@google.com>
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820794}