Cleanup getBBox and getStrokeBBox implementations

The bounding box code in Blink was old and followed the SVG 1.1 spec.
This patch refactors how getBBox and getStrokeBBox are implemented,
moving them off of SVGLocatable and onto SVGGraphicsElement. With
this patch, we no longer route bounding box calls through static
methods on SVGLocatable. This cleans up the code and makes our
implementation match the SVG2 spec:
https://svgwg.org/svg2-draft/single-page.html#types-InterfaceSVGGraphicsElement

This patch also adds a lost semicolon to getStrokeBBox.html

No new tests as this is just a refactoring without a behavior change.

Review URL: https://codereview.chromium.org/47913003

git-svn-id: svn://svn.chromium.org/blink/trunk@160748 bbb929c8-8fbe-4397-9dbb-9b2b20218538