Revert of [SVG] Promote PathOpsSVGClipping to stable. (patchset #1 id:1 of https://codereview.chromium.org/446583002/)

Reason for revert:
We've received valuable feedback from exposure to stable. Dialing back while the issues are being worked on.

Original issue's description:
> [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
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=179529

TBR=fs@opera.com,pdr@chromium.org,schenney@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=263911

Review URL: https://codereview.chromium.org/645023003

git-svn-id: svn://svn.chromium.org/blink/trunk@183894 bbb929c8-8fbe-4397-9dbb-9b2b20218538