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A focusable element can be invisible to user when another
element covers entire the focusable element. In most cases
giving focus to the invisible focusable element is not good
because user cannot know where is focus and what's going on.

Moreover, let's assume that a floating menu is displayed and
we want to navigate items in the menu. In this case, we want
to navigate each item regardless of any focusable element under
the floating menu.

To navigate only visible element, check visibility of candidate.
We treat an element as visible when the element is not offscreen,
and the element is included in hit test result.

Change-Id: Iafc89edd77b05cc047395440548004a937bba8c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1575314
Commit-Queue: JunHo Seo <junho0924.seo@lge.com>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659441}