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Background:
 SpatNav compares activeElement's bounding rect
 with all other focusables' bounding rects.

Example (snav-fragmented-link.html):
 A multi-line ("fragmented") link has focus.

  xxxxxxLINK L
  INKLINKxx #A

 Here LINK's bounding box's area is bigger than the
 the combined area of its two fragments. The box,
 returned by NodeRectInRootFrame(), even overlaps
 another focusable, #A. We can say that LINK's
 bounding box "contains" #A.

Problem:
 Spatial navigation searches _from_ the focused
 element's bounding box (but not inside of it).
 Here LINK's rect completely contains A's rect so
 #A will never be considered as a focus candidate.

Solution:
 Give priority to focusables inside the current focus rect.

 This also solves the generic case, crbug.com/798102,
 where one focusable lays "under" another one, i.e
 where a focus rect "contains" (or is positioned
 behind) another focusable. I test this case in
 snav-focus-rect-contains-links.html.

 Previously, such containers where unreachable because
 of 86d40f3d's (anno 2010):

 "... if there are 2 overlapping focusable nodes, we do
 a hit test and only the node on top can get focus."

 Now, when two focusables overlap, SpatNav first goes
 go the outer box. From there, focus can go to the
 "on-top", or "inner" (thinking in 2D) focusables.

Related CLs:
 Search scrollers from their edges: crrev.com/589502.
 This CL uses the same approach as crrev.com/589502;
 when a "container" is focused, we first search inside of
 it, from one of its edges.

Distance calculation fixup:
 OppositeEdge()'s returned slice will now be "just outside"
 the box it is slicing. This change helps us avoid if-logic
 to cover cases when focusables sit at the very top of their
 container.

Bonus cleanup:
 AreElementsOnSameLine() can now be removed. It was
 introduced back in 2011 at 3568f132 to support
 [some] line-broken links but it didn't fix 956900.

Bug: 798102, 956900
Change-Id: I00aaba88a7846b36275f9913034145679576e410
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1587559
Commit-Queue: Hugo Holgersson <hholgersson@fb.com>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658484}