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This includes:
1. Factoring out preference-specific logic away from the shared SyncTest
   class.
2. Improved failure messages for the case where engine initialization
   fails.
3. Rewrite the test to just test the intended behavior under test,
   which is about two clients having a mismatching preference type.

Bug: 1012688
Change-Id: I7679a6c2b7963bdb181a35e2f327944320781896
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1898655
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712563}