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This CL sets up the infrastructure for private instrumentation tests of
WebLayer's infobars and adds an initial test: that addition of an
infobar impacts the visual state of the infobar container as expected.

BUG=1093846

Change-Id: Id5cdc73f02a4eadb46b3733451541fbd1f796441
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2247836
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779772}