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This CL adds a new accessibility private API where extensions will be
able to listen a ChromeEvent which is dispatched when TalkBack state in
ARC accessibility gets changed.

Android side change is http://ag/10260750.

Bug: 981964
Test: unit_tests --gtest_filter="ArcAccessibilityHelperBridgeTest.*"
Change-Id: I9012e9761d3995bd686c2651ecc9921ed8fc15e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2037187Reviewed-by: Akihiro Ota <akihiroota@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: Hiroki Sato <hirokisato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741688}