Add helper for computing outset for 'outline'

De-duplicates code from InlinePainter and SVGLayoutSupport.

Review URL: https://codereview.chromium.org/1165393005

git-svn-id: svn://svn.chromium.org/blink/trunk@196880 bbb929c8-8fbe-4397-9dbb-9b2b20218538
4 files changed