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Add the AIRA role of "alert" to the "selected file is too big" error
message. This should convince screen readers to announce the error. It's
time sensitive (per aira "alert" role guidelines) because if the user
presses "Send" before hearing the alert, they won't know the file wasn't
sent. Not perfect (if the user chooses another file that is too big, the
error is not re-announced), but better than what we have currently.

Also add aira-describedby to the attach file window, so that the screen
readers treat "File will be sent to Google for debugging" as an extra
description of the field.

BUG=chromium:1005559, chromium:1005528
TEST=ChromeVox on ChromeOS. "Selected file is too big" was announced
after selecting a large file. (Took a long time, but it was announced).
"File will be sent to Google for debugging" was spoken after "Attach
file No file chosen".

R=afakhry@chromium.org

Change-Id: Ic60b811cd1902e6e621048ed6e9ab6da838ba267
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1863363Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Auto-Submit: Ian Barkley-Yeung <iby@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706282}