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When looking for untested parts of the public WebLayer API, found we
don't have tests for ObserverList. This copies the tests from the //base
version.

//base runs these tests by including them in the content_shell
instrumentation tests, so do the same thing here by including them in
weblayer instrumentation tests.

Change-Id: I7cbacf6efdb8c9f0bcdebc96cafdb7274b392d2c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1903664Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713609}