Implement SVG2's getStrokeBBox()

This patch implements getStrokeBBox() from:
https://svgwg.org/svg2-draft/single-page.html#types-__svg__SVGGraphicsElement__getStrokeBBox

The patch is mostly straightforward plumbing of the stroke bounding
box to javascript. This patch also contains a small cleanup removing
the unused StyleUpdateStrategy parameter from getBBox().

BUG=280423

Review URL: https://codereview.chromium.org/46063009

git-svn-id: svn://svn.chromium.org/blink/trunk@160684 bbb929c8-8fbe-4397-9dbb-9b2b20218538