Turn SVG pathops shape union into a runtime enabled feature.

This is a soft revert of https://src.chromium.org/viewvc/blink?revision=154124&view=revision.

While pathops is definitely the way we want to go, there are several outstanding issues that need to be addressed first.

R=pdr@chromium.org,schenney@chromium.org

BUG=262531

Review URL: https://chromiumcodereview.appspot.com/19734010

git-svn-id: svn://svn.chromium.org/blink/trunk@154757 bbb929c8-8fbe-4397-9dbb-9b2b20218538