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When the app opens, the focus is set onto the Scanner dropdown once it
finishes loading.
When a scan starts the focus is set on the Cancel button.
When a scan completes the focus is set on the scanned images.
Returning to the initial page after a completed scan, the focus
returns to the Scanner dropdown.

Bug: 1059779
Change-Id: I937eb956f52ff2a53ffcdd50c38be55138803800
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2581448
Commit-Queue: Gavin Williams <gavinwill@chromium.org>
Reviewed-by: Jesse Schettler <jschettler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835923}