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This also only instantiates the previews opt guide if previews are
allowed.

The root of the flake is that Previews was registering the opt target
which caused the fetch for regular profiles. By disabling the previews
feature in our tests, we are able to better isolate the logic we want
to test. (Previews was default enabled on Linux)

Bug: 1158257
Change-Id: I3bfe66979bb156698e438b26e2bccf6c172dd140
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2590708
Commit-Queue: Sophie Chang <sophiechang@chromium.org>
Reviewed-by: Michael Crouse <mcrouse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836864}