[SVG] Promote PathOpsSVGClipping to experimental.

Re-enabling pathops-based SVG clipping after numerous Skia updates.

A handful of pixel tests require minor rebaselines.

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

Review URL: https://codereview.chromium.org/362333002

git-svn-id: svn://svn.chromium.org/blink/trunk@177411 bbb929c8-8fbe-4397-9dbb-9b2b20218538