Fix flakiness in SingleClientSessionsSyncTest.CookieJarMismatch

Unless something is done about it, there is a race between signin code
trying to fetch "real" cookiejar information using GaiaAuthFetcher, and
the test faking equivalent information and injecting it via
ProfileSyncService::OnGaiaAccountsInCookieUpdatedWithCallback().

Usually, signin logic is fast enough to avoid test failures, but
occasionally the fetching takes long and an empty list of accounts is
injected at a last stage (after navigations to kURL2), interfering with
test expectations.

Bug: 789129
Change-Id: I95b0f691bd047216309888d345c68e1eb26ab869
Reviewed-on: https://chromium-review.googlesource.com/c/1371386
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615504}
1 file changed