Revert "Reland: sync_integration_tests: don't instantiate ScopedFeatureList in test body"

This reverts commit a0f3d164c7bb26b7677cddc30b2e1f740d68542d.

Reason for revert:

Findit (https://goo.gl/kROfz5) identified CL at revision 621523 as the
culprit for flakes in the build cycles as shown on:
https://findit-for-me.appspot.com/waterfall/flake/flake-culprit?key=ag9zfmZpbmRpdC1mb3ItbWVyQwsSDEZsYWtlQ3VscHJpdCIxY2hyb21pdW0vYTBmM2QxNjRjN2JiMjZiNzY3N2NkZGMzMGIyZTFmNzQwZDY4NTQyZAw

Sample Failed Build: https://ci.chromium.org/buildbot/chromium.memory/Linux%20TSan%20Tests/32409

Sample Failed Step: sync_integration_tests

Sample Flaky Test: SingleClientWalletWithAccountStorageSyncTest.SwitchesBetweenAccountAndProfileStorageOnTogglingSync

Original change's description:
> Reland: sync_integration_tests: don't instantiate ScopedFeatureList in test body
> 
> This is a reland of https://crrev.com/c/1402799. The original CL was
> reverted for supposedly introducing flakiness, but in fact in only
> renamed an already-flaky test. That test has since been disabled, so
> trying again.
> 
> 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: Iaa6185d0dd9a042a9275100e3b1cae1eceb52633
> Reviewed-on: https://chromium-review.googlesource.com/c/1402890
> Reviewed-by: Mikel Astiz <mastiz@chromium.org>
> Commit-Queue: Marc Treib <treib@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#621523}

Change-Id: I3c8804e10fc6fd5c15e540f4b0441926da421634
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 915219, 920738
Reviewed-on: https://chromium-review.googlesource.com/c/1405984
Cr-Commit-Position: refs/heads/master@{#621737}
5 files changed