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Previously, to detect if VoiceOver was running on the Mac, we used an
undocumented attribute (AXEnhancedUserInterface) that was set on the
application.  Unfortunately, this doesn't work with the new VoiceControl
system.

After discussions with Apple, they recommended that we turn on a11y
whenever an AT accesses the 'accessibilityRole' property.  This would
work for VoiceControl, and should work with any new ATs in the future.
Since this attribute is called frequently, we use 'dispatch_once' to
ensure the expensive work is only done once.

One disadvantage is that we can no longer turn *off* a11y when the
'AXEnhancedUserInterface' attribute is set to zero, because doing so
breaks compatibility with VoiceControl.  (A11y would be enabled, then
quickly disabled again.)

Bug: 1143047
Change-Id: I41ed07b31476cef4a04fd0f2d9c994acc9311654
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2538257Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Ian Prest <iapres@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#833149}