Remove the visibility check in SVGAElement::rendererIsFocusable()

This patch removes a check that an SVGAElement must have a
non-empty clipped overflow rect to be focusable.

wkrev.com/26398[1] added a check that an SVG <a> element
must be visible to be focusable. This is not what other browsers
support (IE focuses off-screen SVG <a> elements, Firefox doesn't
focus SVG <a> elements at all). The SVG2 spec has clarified the
wording around focusable to explicitly state: "Just being
off-screen does not mean the element is not being rendered"[2]
when referring to whether an element should be focusable.

[1] http://trac.webkit.org/changeset/26398
[2] https://svgwg.org/svg2-draft/single-page.html#interact-focusable

Review URL: https://codereview.chromium.org/138793005

git-svn-id: svn://svn.chromium.org/blink/trunk@166542 bbb929c8-8fbe-4397-9dbb-9b2b20218538