[SVG] Promote PathOpsSVGClipping to stable.

The feature has been in experimental state for more than a month with
no known issues.

BUG=263911
R=schenney@chromium.org, fs@opera.com, pdr@chromium.org

Review URL: https://codereview.chromium.org/446583002

git-svn-id: svn://svn.chromium.org/blink/trunk@179529 bbb929c8-8fbe-4397-9dbb-9b2b20218538