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 Scenario for bug:
  1. The focused element F is inside a scrollable area A.
  2. F is offscreen (=clipped) but A is (partly) visible.

 Problem:
  Spatnav started to search from the document's edge, not
  from A's edge.

 Solution:
  Set the "search origin" to one of A's outer edges.

To solve this problem we redefine the "search origin" of
spatial navigation. The search origin is either activeElement
itself, if it's being at least partially visible, or its
first [partially] visible scroller.

If both F and its enclosing scroller A are completely
offscreen, we recurse to the scroller’s scroller up until
the root frame's document. The root document is a good base
case because it's, per definition, a visible scrollable area.

This builds onto the idea of:
https://chromium-review.googlesource.com/c/chromium/src/+/873645

Bug: 804669

Change-Id: I15b0f41426d7632fe9ec62d77a414d592e3631c0
Reviewed-on: https://chromium-review.googlesource.com/883533
Commit-Queue: Hugo Holgersson <hugoh@vewd.com>
Reviewed-by: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#589502}