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This CL changes the aria label from simply "Back." to something more
descriptive to the screen reader user: "[Subpage title] back button",
with the role description: "Click to navigate away from [Subpage title]
page" so that the user is aware that they are currently at the
"[name of subpage]" subpage, and that activating the back button will
cause a route change to the previous page.

Videos:
https://drive.google.com/drive/folders/14JjstEYIC3-la2gIlGzo1pRa__NXDYBm?usp=sharing

Bug: 1071284
Change-Id: I4bd6e4400c84b63fe89ad2fe68f62e6d6f468657
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2200144
Commit-Queue: Regan Hsu <hsuregan@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770852}