Update previews_browsertest to use best practices

Specifically, EmbeddedTestServer should be recreated for every test and
only started after the browser process. Also reinitialize
ScopedFeatureList with every test as per recommended in header file.

Bug: 831838
Change-Id: I3e90d69b2d7708ca4abd73577972c97f119c1ab1
Reviewed-on: https://chromium-review.googlesource.com/1018109
Commit-Queue: Tim Brown <timbrown@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551955}
1 file changed