Implementation for penetrating image selection experiment.

This change updates image selection behavior to consider elements below transparent non-image elements, with certain restrictions.
The change is protected behind a finch feature, `EnablePenetratingImageSelection`, so we can measure impact before enabling.

Change-Id: I9cf6f137ba4231cd12ddcc574d1ec6d2f41337a9
Bug: 1177459
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2677528
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Filip Gorski <fgorski@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Ben Goldberger <benwgold@google.com>
Cr-Commit-Position: refs/heads/master@{#861288}