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/tests/PaintTest.cpp b/tests/PaintTest.cpp
index 7affa9e..a58bd86 100644
--- a/tests/PaintTest.cpp
+++ b/tests/PaintTest.cpp
@@ -146,8 +146,7 @@
     paint.setStrokeWidth(SkIntToScalar(2));
     // set a few pointers
     SkLayerDrawLooper::Builder looperBuilder;
-    SkLayerDrawLooper* looper = looperBuilder.detachLooper();
-    paint.setLooper(looper)->unref();
+    paint.setLooper(looperBuilder.detach());
     SkMaskFilter* mask = SkBlurMaskFilter::Create(kNormal_SkBlurStyle,
                                       SkBlurMask::ConvertRadiusToSigma(SkIntToScalar(1)));
     paint.setMaskFilter(mask)->unref();