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Before this change, element coordinates were relative to their window,
i.e. frame. For elements in iFrame this meant that the "cutout" was
misplaced.
This change now keeps a frame-stack while retrieving elements and
then sums up the position to get a "global" position of the element
for using in a showcast.

Bug: b/143942385
Change-Id: I8f3827002df58874d7921e969954f911630cc4ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2074499
Commit-Queue: Sandro Maggi <sandromaggi@google.com>
Reviewed-by: Mathias Carlen <mcarlen@chromium.org>
Reviewed-by: Stephane Zermatten <szermatt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745566}