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Previously, the tests waited for action_timeout(), to wait for LBS to
load the rules and write some files. As expected, this is fairly flaky,
and it slows down the tests.

Now, `BrowserSwitcherService' exposes a new event for testing, which
fires once LBS has loaded the rules. This should reduce flakes by
listening for the event instead of using an unreliable timeout.

Also rewrote some tests in a synchronous style, which improves
readability.

Bug: 1044619, 1070400
Change-Id: If1a2093cfe3bface73a46896ff2bc95b80ab2f2e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2152885
Commit-Queue: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#760531}