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/samplecode/SampleLayers.cpp b/samplecode/SampleLayers.cpp
index 4386976..52e6593 100644
--- a/samplecode/SampleLayers.cpp
+++ b/samplecode/SampleLayers.cpp
@@ -255,7 +255,7 @@
         m.postTranslate(fCenter.x(), fCenter.y());
         path.transform(m);
 
-        canvas->clipPath(path, SkRegion::kIntersect_Op, true);
+        canvas->clipPath(path, SkCanvas::kIntersect_Op, true);
         const SkRect bounds = path.getBounds();
 
         SkPaint paint;