Add tabindex handling in svg, by reusing code from html.

Besides adding the tabindex attribute also reflect it to the tabIndex IDL attribute on the SVGElement interface.

Spec: https://svgwg.org/svg2-draft/interact.html#sequential-focus-navigation-and-the-tabindex-attribute
IDL attribute: https://svgwg.org/svg2-draft/types.html#__svg__SVGElement__tabIndex

BUG=352175

Review URL: https://codereview.chromium.org/166163005

git-svn-id: svn://svn.chromium.org/blink/trunk@172964 bbb929c8-8fbe-4397-9dbb-9b2b20218538