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This change updates tooltip for the latest design spec.
The tooltips describe either the number of notifications or being in
DnD mode, so that these should work well for a11y label as well.

Bug: 888072
Change-Id: Ifed382eca722c6948cf14a9f834ad0bb314c1629
Reviewed-on: https://chromium-review.googlesource.com/c/1306935Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Commit-Queue: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603784}