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At one point, the document provider expected responses with http status
codes of 200 with a specific error code in the response body. This was
deprecated in favor of responses with 400 & 499 http status codes. For a
while, the provider handled both. Now that the backend exclusively
employs the new responses, it's safe to remove the original 200 backoff
handling.

Change-Id: Ie79fedaa6e0930db3f32aefedd3ccde699f984a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2416561Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Commit-Queue: manuk hovanesian <manukh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808465}