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This is a reland of 291faaf5

The original version of the test was flaky. The bug was in assuming the
entities are ordered by arrival time. They are actually ordered by
some unique id, which is random, and so the order was "wrong" about half
of the time.

Original change's description:
> [SyncInvalidations] Add signout and signin integration test.
>
> This CL adds a test that checks the following:
> - On signout, the FCM registration token should be cleared.
> - On signin, it should be requested again.
>
> Bug: 1135167
> Change-Id: Ie1b1967b9ed317c0a36d4b751aef0dfeb81c45d8
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2467979
> Commit-Queue: Paula Vidas <paulavidas@google.com>
> Reviewed-by: Marc Treib <treib@chromium.org>
> Reviewed-by: Rushan Suleymanov <rushans@google.com>
> Cr-Commit-Position: refs/heads/master@{#817408}

Bug: 1135167
Change-Id: I5ea7c873d6782bf53ccff040d956c2a03845acd3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2485105
Commit-Queue: Paula Vidas <paulavidas@google.com>
Reviewed-by: Rushan Suleymanov <rushans@google.com>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818840}