Fix flaky test: Scoped feature list to be changed in SetUp

This Cl fixes the new history intervention browser tests to not be flaky.
ScopedFeatureList should only be changed in the SetUp method and not in
the individual tests.

Bug: 920022
Change-Id: I8b442b94fea844aba2b85b940ddfee0f65b80372
Reviewed-on: https://chromium-review.googlesource.com/c/1403178
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Commit-Queue: Charlie Reis <creis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621226}
1 file changed