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This change adds an ARIA label for the preview area that is updated
depending on the current page:
 -In the initial state, grab the scan preview instructions from the page
 -While scanning, update the label when the page number increases
 -When the scan is completed successfully, announce scan is completed
  and the total number of pages scanned.

Bug: 1059779
Change-Id: I1fcdf2552f779737a69d95fe5971c1b98c0a6aa5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2580846
Commit-Queue: Gavin Williams <gavinwill@chromium.org>
Reviewed-by: Jesse Schettler <jschettler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835882}