Fix flakiness in OAuth2TokenServiceTest.java

This fixes at least part of the "Flakiness": CL modifying the code
introduced a real bug in the test, as it should not fail automatically
within |onGetTokenFailure| and it should wait for the execution to
complete.

I ran the tests several times, both locally and on try bots without any failure.

Bug: 962976
Change-Id: Id4b88380cfdaaa31445c8ae0ed74956f54ea17d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1611993
Commit-Queue: Pâris Meuleman <pmeuleman@chromium.org>
Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662092}
1 file changed