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Before this change, CheckOnTop would only check the current frame. While
this could detect the case where an element covers the goal element,
this did not detect the case where an element covers the iframe
containing the element.

With this change, CheckOnTop requires the target element as well as any
iframe containing the target element to be on top (not covered by
another element).

Bug: b/171463353
Change-Id: I3d32fdff3cb55724fd096de18034e9b4e002cab3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2523172
Commit-Queue: Stephane Zermatten <szermatt@chromium.org>
Reviewed-by: Sandro Maggi <sandromaggi@google.com>
Cr-Commit-Position: refs/heads/master@{#825777}