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This CL adds a feature pod button for accessibility. By clicking on the
button, it will show accessibility detailed view of the existing
SystemTray. This behavior is described as Development Stage 1 in the
design doc: go/cros-qs-restyling

Screenshot: http://screen/scc32RkykN2

TEST=AccessibilityFeaturePodControllerTest
BUG=821671

Change-Id: I2c9f91b86c70e5bad3c1eb4261c14e4d167f0473
Reviewed-on: https://chromium-review.googlesource.com/961923
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543041}