Change RenderSVGResourceClipper's ClipperContext class into an enum

RenderSVGResourceClipper needs a couple bits of state to determine
whether mask or path clipping was used. (The clipper doesn't maintain
this state because it can be applied to multiple renderers.) This patch
converts the ClipperContext class into a simple enum which is now
scoped to RenderSVGResourceClipper.

No new tests as there is no change in behavior.

Review URL: https://codereview.chromium.org/547723003

git-svn-id: svn://svn.chromium.org/blink/trunk@181575 bbb929c8-8fbe-4397-9dbb-9b2b20218538