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/SampleIdentityScale.cpp b/samplecode/SampleIdentityScale.cpp
index b6af3ce..8d1bc36 100644
--- a/samplecode/SampleIdentityScale.cpp
+++ b/samplecode/SampleIdentityScale.cpp
@@ -65,7 +65,7 @@
           SkRect r = { 100, 100, 356, 356 };
           SkPath clipPath;
           clipPath.addRoundRect(r, SkIntToScalar(5), SkIntToScalar(5));
-          canvas->clipPath(clipPath, SkRegion::kIntersect_Op, SkToBool(1));
+          canvas->clipPath(clipPath, SkCanvas::kIntersect_Op, SkToBool(1));
           text = "Scaled = 0";
         }
         canvas->drawBitmap( fBM, 100, 100, &paint );