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Currently the field is only labelled by its placeholder text, which
is not read by all screenreaders. This change gives it an explicit
accessible label instead.

Bug: 1083307
Change-Id: I7b79de41576c9a9c4caf33ef54a5009329d3d0f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2538531Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832153}