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This change fixes an infinite loop in `OneShotAccessibilityTreeSearch`.
If the search starts at the root of a tree, goes backwards, and wraps
around to the last node, it will loop forever in `SearchByWalkingTree`.

The problem is that `stop_node` is set to NULL, so it is never matched.

The fix is to ensure that stop_node gets set to the root element (i.e.,
the start_node) so that we don't loop through the tree more than once.

Bug: 1001200
Change-Id: I67d8abd5e6044b2a125a633d39c7581e45cc7411
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1790255Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Kevin Babbitt <kbabbitt@microsoft.com>
Commit-Queue: Ian Prest <iapres@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#695764}