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We add integration test coverage for PREFERENCES under pseudo-USS, and
factor out some duplicated code to a newly introduced FeatureToggler, a
helper class to run tests with a feature switch enabled and disabled.

Bug: 870624
Change-Id: I6b1b446439969d5c6e948e21ca160876e14c5cdb
Reviewed-on: https://chromium-review.googlesource.com/1253762
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595740}