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/aaclip.cpp b/gm/aaclip.cpp
index cb8418b..d657669 100644
--- a/gm/aaclip.cpp
+++ b/gm/aaclip.cpp
@@ -85,7 +85,7 @@
     canvas->drawRect(target, borderPaint);
     target.inset(SkIntToScalar(2), SkIntToScalar(2));
     canvas->drawRect(target, backgroundPaint);
-    canvas->clipRect(target, SkRegion::kIntersect_Op, true);
+    canvas->clipRect(target, SkCanvas::kIntersect_Op, true);
     target.inset(SkIntToScalar(-4), SkIntToScalar(-4));
     canvas->drawRect(target, foregroundPaint);
     canvas->restore();