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The underlying issue is that by the time we get around to processing a live
region node from a mutation observer, the node may no longer be part of
the DOM, in which case we should treat it as invisible.

To fix it, fix cvox.DomUtil.hasInvisibleAncestor_ so that it returns
true (invisible) if passed an element that's not actually part of the
document.

BUG=408821

Review URL: https://codereview.chromium.org/517853002

Cr-Commit-Position: refs/heads/master@{#292847}