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/src/effects/SkLayerDrawLooper.cpp b/src/effects/SkLayerDrawLooper.cpp
index 83f21e5..ae31482 100644
--- a/src/effects/SkLayerDrawLooper.cpp
+++ b/src/effects/SkLayerDrawLooper.cpp
@@ -230,7 +230,7 @@
         info.fPostTranslate = buffer.readBool();
         buffer.readPaint(builder.addLayerOnTop(info));
     }
-    return builder.detachLooper();
+    return builder.detach().release();
 }
 
 #ifndef SK_IGNORE_TO_STRING
@@ -344,7 +344,7 @@
     return &rec->fPaint;
 }
 
-SkLayerDrawLooper* SkLayerDrawLooper::Builder::detachLooper() {
+sk_sp<SkDrawLooper> SkLayerDrawLooper::Builder::detach() {
     SkLayerDrawLooper* looper = new SkLayerDrawLooper;
     looper->fCount = fCount;
     looper->fRecs = fRecs;
@@ -353,5 +353,5 @@
     fRecs = nullptr;
     fTopRec = nullptr;
 
-    return looper;
+    return sk_sp<SkDrawLooper>(looper);
 }