guard rasterizer and drawlooper setters

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

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1821533002

Review URL: https://codereview.chromium.org/1821533002
diff --git a/samplecode/SampleSlides.cpp b/samplecode/SampleSlides.cpp
index ca0d9df..9b054ad 100644
--- a/samplecode/SampleSlides.cpp
+++ b/samplecode/SampleSlides.cpp
@@ -544,7 +544,7 @@
     p.setAntiAlias(false);
     SkLayerRasterizer::Builder rastBuilder2;
     r5(&rastBuilder2, p);
-    p.setRasterizer(rastBuilder2.detachRasterizer())->unref();
+    p.setRasterizer(rastBuilder2.detach());
     p.setXfermodeMode(SkXfermode::kClear_Mode);
     rastBuilder->addLayer(p);
 }
@@ -615,7 +615,7 @@
 
     p.setAntiAlias(true);
     proc(&rastBuilder, p);
-    paint->setRasterizer(rastBuilder.detachRasterizer())->unref();
+    paint->setRasterizer(rastBuilder.detach());
     paint->setColor(SK_ColorBLUE);
 }