Fix data races in SingleClientCustomPassphraseSyncTest

Currently these tests are all very flaky on TSan because of a data race
related to the global FeatureList instance. This CL changes the tests
to avoid the problematic pattern (overriding the FeatureList from the
test body).
For one test (CannotDecryptScryptKeyEncryptedDataWhenScryptDisabled),
it's not easy to avoid the bad pattern, so this test is disabled on
TSan.

Bug: 915219
Change-Id: Ic04db73c6a40b426f52fbf4a170d661c2a1ebe93
Reviewed-on: https://chromium-review.googlesource.com/c/1404084
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621960}
2 files changed