capture_mode: Improve test coverage of the recording service

The recording service had some test coverage as follows:
  - Many browser tests that verifies recording end-to-end, and
    asserts the produced webm files are valid.
  - Many unit tests in ash_unittests that used an instance of
    a fake service, just to verify that ash sends it the right
    events via its mojo APIs.

However, this did not allow us to test the *real* recording
service in unit tests, and be able to poke into its internals.
This lead to a serious regression (https://crbug.com/1214023)
in window recording which could not be caught by any of our
tests. That's due to the fact that catching such regression
requires intercepting the received video frames from the Viz
capturer and verifying them.

This CL fixes this by getting rid of the fake service, and
instead creates an in-process instance of the *real* service
that can be tested via a newly-added testing APIs.

BUG=1215819
TEST=Existing unittests. Added a new regression test for
     https://crbug.com/1214023 that currently fails without
     the fix, and hence disabled. This verifies the new
     testing infra structure works as intended.

Change-Id: I144b4441bc4a31c7a02d4850281e2c9452fd7c59
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2939182
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#889412}