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Deregistration from old invalidations is moved from the constructor to
the Initialize() method. The change is needed because
invalidator_->RegisterInvalidationHandler() calls
OnInvalidatorClientIdChange(), which cannot be called before the
SyncEngineImpl is initialized, since it uses the sync_task_runner_.

CreateBacked() helper is removed from the tests because it's not needed
anymore.

Bug: 1082115
Change-Id: I2846acf63eb0d245576b5ec65a435b522db05a21
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2442969
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@{#813150}