Current heuristic spatial navigation algorithm has significant issues
with overlapping elements. The algorithm ignores the elements that
have non-empty intersection with current element, even if the intersection
is in its transparent areas which may give an effect of elements of menu
being skipped at some fancier sites.

The algorithm for selecting neighbouring element was fixed as well
as the distance calculation to match exactly the formula from
http://www.w3.org/TR/WICD/#focus-handling which also proved to improve
the user experience.

BUG=

Review URL: https://codereview.chromium.org/105973003

git-svn-id: svn://svn.chromium.org/blink/trunk@170945 bbb929c8-8fbe-4397-9dbb-9b2b20218538