[css-nav-1] Define points precisely for measuring the distance

[jihyerish]

In the selecting the best candidate step, the distance is measured for each candidate from the starting point.
But it's unclear that which points of the boundary box area are chosen. (for P1 and P2)
It would be
(1) the center point of the boundary box
(2) the center point of edges of each element
(3) the closest point along the edges of each element (Implementation in Blink)

• Related issue : Spatial Navigation w3ctag/design-reviews#287

[jihyerish]

I'd like to suggest selecting the point as the closest point along the edges of each element (Implementation in Blink).

Selecting the point from the center of an element is also considerable, but it cannot solve the problem when candidate elements are aligned with the same Euclidean distance from the currently focused element.

I've implemented test cases in UX point of view, and tested for several ways to select points.
Please see the result in here.