sync_integration_tests: don't instantiate ScopedFeatureList in test body

Creating a ScopedFeatureList instance in the test body creates a data
race related to the global FeatureList instance: The ScopedFeatureList
overrides the global instance, and then it gets reset after the test
body; however, the feature list might still be queried from the Sync
thread after that.

This CL avoids the issue by instead creating test (sub)classes to hold
the ScopedFeatureLists.

Bug: 915219
Change-Id: Ib1e0d87ed4ecc5ef33e9601ba84efecd399fb003
Reviewed-on: https://chromium-review.googlesource.com/c/1402799
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621134}
5 files changed