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This CL implements a basic hit testing system for finding links
associated with rects. An instance of the Hit Tester will exist for each
frame to address stacking.

This may return multiple links per query. In such cases, it is up to
the caller to decide which link if any to resolve.

Bug: 1012891
Change-Id: I772ebecae7a7403228e6c50cdc499c1ed7690fcd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1849209
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Reviewed-by: Ian Vollick <vollick@chromium.org>
Reviewed-by: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706085}