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

gm/aaclip.cpp

Index: gm/aaclip.cpp
diff --git a/gm/aaclip.cpp b/gm/aaclip.cpp
index d6576691ea9e0dee915939efa00fe51479a94670..26a25c13856878383abe80318c99c896220d60ff 100644
--- a/gm/aaclip.cpp
+++ b/gm/aaclip.cpp
@@ -85,7 +85,7 @@ static void draw(SkCanvas* canvas, SkRect& target, int x, int y) {
     canvas->drawRect(target, borderPaint);
     target.inset(SkIntToScalar(2), SkIntToScalar(2));
     canvas->drawRect(target, backgroundPaint);
-    canvas->clipRect(target, SkCanvas::kIntersect_Op, true);
+    canvas->clipRect(target, true);
     target.inset(SkIntToScalar(-4), SkIntToScalar(-4));
     canvas->drawRect(target, foregroundPaint);
     canvas->restore();