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Since we moved to primarily walking the DOM tree, we are automatically
getting aria-hidden=false elements in the tree, so we don't need to
specially add them.

Change-Id: Id44b451dc47e3baa301258254d89edb7a2bbe14e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2545846Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Alice Boxhall <aboxhall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829026}