I just had a chat with Pavel and In-Ho about this. The sync server will not send down tombstones for first-time syncs, but we should still:

1. Add code in the processor to filter them out.
2. Ensure the migration code doesn't produce them.

Otherwise we will likely see issues with this in the future.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/23a04c49a42e8ddef2bcc9755f02038840409475

commit 23a04c49a42e8ddef2bcc9755f02038840409475
Author: maxbogue <maxbogue@chromium.org>
Date: Wed Jan 18 01:11:16 2017

[Sync] USS: Filter out tombstones for initial sync.

Also prevents migration from sending tombstones.

BUG= 680550 

Review-Url: https://codereview.chromium.org/2630613002
Cr-Commit-Position: refs/heads/master@{#444219}

[modify] https://crrev.com/23a04c49a42e8ddef2bcc9755f02038840409475/components/sync/engine_impl/test_entry_factory.cc
[modify] https://crrev.com/23a04c49a42e8ddef2bcc9755f02038840409475/components/sync/engine_impl/test_entry_factory.h
[modify] https://crrev.com/23a04c49a42e8ddef2bcc9755f02038840409475/components/sync/engine_impl/uss_migrator.cc
[modify] https://crrev.com/23a04c49a42e8ddef2bcc9755f02038840409475/components/sync/engine_impl/uss_migrator_unittest.cc
[modify] https://crrev.com/23a04c49a42e8ddef2bcc9755f02038840409475/components/sync/model/fake_model_type_sync_bridge.cc
[modify] https://crrev.com/23a04c49a42e8ddef2bcc9755f02038840409475/components/sync/model_impl/shared_model_type_processor.cc
[modify] https://crrev.com/23a04c49a42e8ddef2bcc9755f02038840409475/components/sync/model_impl/shared_model_type_processor_unittest.cc