[SVG2] css 'outline' property should apply to svg elements

The 'outline' property was only partially working in svg before this patch,
this makes it work on text and text content child elements too.

This makes SVG render the outlines as part of the foreground paint phase.

Partly based on Florin Malita's webkit patch https://bugs.webkit.org/show_bug.cgi?id=113666#c12.

BUG=369473

Review URL: https://codereview.chromium.org/261773008

git-svn-id: svn://svn.chromium.org/blink/trunk@174186 bbb929c8-8fbe-4397-9dbb-9b2b20218538