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This change fixes an issue where giveChromeVoxHint_ was being called
several times. This is likely because it's bound to an event listener,
which isn't removed until several calls have been queued up. To
prevent this, use a variable to track when the hint is given.

Fixed: 1162246
Change-Id: I9e9ed3ac5ce83e7011d29b2edc392d019289803a
AX-Relnotes: N/A
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2613366Reviewed-by: Denis Kuznetsov [CET] <antrim@chromium.org>
Commit-Queue: Akihiro Ota <akihiroota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841541}