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The distance calculation was broken due to pinned element and
hovered element used different coordinates. r91bef92b also fixed
this issue. This CL refactored the function name to make the
coordinates used clearer, and also added unit test to prevent
future breakage.

BUG=899001

Change-Id: Ic1e2e577eb085a82f5ea32f608312335d6befd42
Reviewed-on: https://chromium-review.googlesource.com/c/1372551Reviewed-by: Leonard Grey <lgrey@chromium.org>
Commit-Queue: Wei Li <weili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615684}