Make ExtensionEventObserverTest a RenderViewHostTestHarness

This simplifies the test harness and makes the test pass with
single process Mash (due to extra setup performed by the parent test
fixture).

Bug: 905430
Change-Id: Ic68c5900bc7632dc9177e730706007fb4e3eab39
Reviewed-on: https://chromium-review.googlesource.com/c/1340949
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609354}
1 file changed