move setshader to sk_sp, re-using SK_SUPPORT_LEGACY_CREATESHADER_PTR

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1829303002

Review URL: https://codereview.chromium.org/1829303002
diff --git a/src/utils/SkLua.cpp b/src/utils/SkLua.cpp
index c25bbf4..01bf507 100644
--- a/src/utils/SkLua.cpp
+++ b/src/utils/SkLua.cpp
@@ -1146,7 +1146,7 @@
 
 static int lpaint_setShader(lua_State* L) {
     SkPaint* paint = get_obj<SkPaint>(L, 1);
-    paint->setShader(get_ref<SkShader>(L, 2));
+    paint->setShader(sk_ref_sp(get_ref<SkShader>(L, 2)));
     return 0;
 }