[Sync::USS] Remove DCHECK(!model_error_) from Processor.GetLocalChanges

This doesn't seem a legit DCHECK, since the error may take time to
propagate to the worker.

Change-Id: Ifd8a4bbc8b9d7691fa891e30678e9cdb56717541
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1571689
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652098}
2 files changed