guard rasterizer and drawlooper setters

Need to land https://codereview.chromium.org/1823513002/ first

BUG=skia:
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Review URL: https://codereview.chromium.org/1821533002
diff --git a/samplecode/ClockFaceView.cpp b/samplecode/ClockFaceView.cpp
index f5b43fb..80debeb 100644
--- a/samplecode/ClockFaceView.cpp
+++ b/samplecode/ClockFaceView.cpp
@@ -163,7 +163,7 @@
 
     p.setAntiAlias(true);
     r7(&rastBuilder, p, scale);
-    paint->setRasterizer(rastBuilder.detachRasterizer())->unref();
+    paint->setRasterizer(rastBuilder.detach());
 
     paint->setColor(SK_ColorBLUE);
 }