allow clip calls w/o op param, remove unnecessary kReplace ops

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2357333002

Review-Url: https://codereview.chromium.org/2357333002
diff --git a/gm/rrects.cpp b/gm/rrects.cpp
index 46c1c67..1f47f77 100644
--- a/gm/rrects.cpp
+++ b/gm/rrects.cpp
@@ -124,7 +124,7 @@
 #endif
                     } else if (kBW_Clip_Type == fType || kAA_Clip_Type == fType) {
                         bool aaClip = (kAA_Clip_Type == fType);
-                        canvas->clipRRect(fRRects[curRRect], SkCanvas::kReplace_Op, aaClip);
+                        canvas->clipRRect(fRRects[curRRect], aaClip);
                         canvas->drawRect(kMaxTileBound, paint);
                     } else {
                         canvas->drawRRect(fRRects[curRRect], paint);