abstract name of clipping ops, to transtion to a more restricted set

SkRegion::Op --> SkCanvas::ClipOp (alias) --> SkClipOp

pre-CL needed in chrome : https://codereview.chromium.org/2355583002/

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2355483002

Review-Url: https://codereview.chromium.org/2355483002
diff --git a/gm/pathopsskpclip.cpp b/gm/pathopsskpclip.cpp
index 4e575ca..77ba84c 100644
--- a/gm/pathopsskpclip.cpp
+++ b/gm/pathopsskpclip.cpp
@@ -42,9 +42,9 @@
             SkIntToScalar(700)
         };
         p.addRoundRect(r, SkIntToScalar(50), SkIntToScalar(50));
-        rec->clipPath(p, SkRegion::kIntersect_Op, true);
+        rec->clipPath(p, SkCanvas::kIntersect_Op, true);
         rec->translate(SkIntToScalar(250), SkIntToScalar(250));
-        rec->clipPath(p, SkRegion::kIntersect_Op, true);
+        rec->clipPath(p, SkCanvas::kIntersect_Op, true);
         rec->drawColor(0xffff0000);
         sk_sp<SkPicture> pict(recorder.finishRecordingAsPicture());