change flattenable factory to return sk_sp

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1855733002

Review URL: https://codereview.chromium.org/1855733002
diff --git a/src/effects/SkLayerDrawLooper.cpp b/src/effects/SkLayerDrawLooper.cpp
index b3430bb..ffcaa43 100644
--- a/src/effects/SkLayerDrawLooper.cpp
+++ b/src/effects/SkLayerDrawLooper.cpp
@@ -214,7 +214,7 @@
     }
 }
 
-SkFlattenable* SkLayerDrawLooper::CreateProc(SkReadBuffer& buffer) {
+sk_sp<SkFlattenable> SkLayerDrawLooper::CreateProc(SkReadBuffer& buffer) {
     int count = buffer.readInt();
 
     Builder builder;
@@ -229,7 +229,7 @@
         info.fPostTranslate = buffer.readBool();
         buffer.readPaint(builder.addLayerOnTop(info));
     }
-    return builder.detach().release();
+    return builder.detach();
 }
 
 #ifndef SK_IGNORE_TO_STRING