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When a download starts or finishes, notify screen reader users
with a friendly message that contains useful information such
as the size and name of the file to be downloaded.

In addition, for long downloads, provide screen reader update
notifications every 30 seconds, and include the time or % remaining.

Bug: 787139
Change-Id: I903dd626fddad16504079c69d4e1f3edd88d8052
Reviewed-on: https://chromium-review.googlesource.com/947769
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541674}