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* Add a function StartScan to the lorgnette_manager_client API that
  allows scanning images using lorgnette's StartScan function.
  - Clients of the API can pass a RepeatingCallback<void(int)>, which
    will be invoked whenever the scan progress has updated.

* Implement StartScan for fake_lorgnette_manager_client.
* Update document_scan_api and unittests to use StartScan instead of
  ScanImageToString.

Bug: b:158504381
Change-Id: I6ca9e7645dc47a72d517d3d7480ea0f12e840e0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2332852Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Jesse Schettler <jschettler@chromium.org>
Commit-Queue: Fletcher Woodruff <fletcherw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795215}