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This moves a bunch of helper functions around secondary account support
from single_client_secondary_account_sync_test.cc into their own files.
This is in preparation for adding more integration tests that will make
use of these.

Bug: 885211
Change-Id: I1f30b2425951933f6f9251f35c4aafb7e1cdb663
Reviewed-on: https://chromium-review.googlesource.com/1233704Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592761}