return pictures as sk_sp

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1811703002

Review URL: https://codereview.chromium.org/1811703002
diff --git a/src/utils/SkLua.cpp b/src/utils/SkLua.cpp
index 868a825..56e2b41 100644
--- a/src/utils/SkLua.cpp
+++ b/src/utils/SkLua.cpp
@@ -1824,12 +1824,12 @@
 }
 
 static int lpicturerecorder_endRecording(lua_State* L) {
-    SkPicture* pic = get_obj<SkPictureRecorder>(L, 1)->endRecording();
-    if (nullptr == pic) {
+    sk_sp<SkPicture> pic = get_obj<SkPictureRecorder>(L, 1)->finishRecordingAsPicture();
+    if (!pic) {
         lua_pushnil(L);
         return 1;
     }
-    push_ref(L, pic)->unref();
+    push_ref(L, std::move(pic));
     return 1;
 }