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This CL adds support for outputting the current progress of a view
progress bar by ChromeVox, which currently says "progress indicator"
without indicating the actual progress.

The logic here is mostly copied from views::Slider.

Bug: 1021414
Change-Id: I9a31708d34b9823ed72a5b9f44b9b08de4df95a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2129367Reviewed-by: Trent Apted <tapted@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Timothy Loh <timloh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758668}