hide setImageFilter(ptr)

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4184

Change-Id: Iad792cfdf54087ad4c424fd268559c162a9a5f5c
Reviewed-on: https://skia-review.googlesource.com/4184
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
diff --git a/src/utils/SkLua.cpp b/src/utils/SkLua.cpp
index 14f074c..a223456 100644
--- a/src/utils/SkLua.cpp
+++ b/src/utils/SkLua.cpp
@@ -1102,7 +1102,7 @@
 
 static int lpaint_setImageFilter(lua_State* L) {
     SkPaint* paint = get_obj<SkPaint>(L, 1);
-    paint->setImageFilter(get_ref<SkImageFilter>(L, 2));
+    paint->setImageFilter(sk_ref_sp(get_ref<SkImageFilter>(L, 2)));
     return 0;
 }