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Add a new ScanJobObserver to receive updates for in-progress scan jobs.
The observer is implemented by and passed from the scan app to the
browser in calls to ScanService::StartScan().

Bug: 1059779
Change-Id: Ic9e73d688f7639cf8a892e8f5d269840d1b981c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2508435
Commit-Queue: Jesse Schettler <jschettler@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825078}