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- make each device contents an ordered list so we get "list 5 items" spoken.
- add a title to each ContextMenuButton so "Actions" is said instead of just "button".
- remove "required" from search input as it is spoken and unnecessary.
- create a useful summary aria-label for checkboxes so users can scroll quickly through entries

R=dmazzoni@chromium.org
BUG=385344

Review URL: https://codereview.chromium.org/664863004

Cr-Commit-Position: refs/heads/master@{#300551}