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/SkMorphologyImageFilter.cpp b/src/effects/SkMorphologyImageFilter.cpp
index dbb116e..bf8811b 100644
--- a/src/effects/SkMorphologyImageFilter.cpp
+++ b/src/effects/SkMorphologyImageFilter.cpp
@@ -72,18 +72,20 @@
     return src.makeOutset(SkScalarCeilToInt(radius.x()), SkScalarCeilToInt(radius.y()));
 }
 
-SkFlattenable* SkErodeImageFilter::CreateProc(SkReadBuffer& buffer) {
+sk_sp<SkFlattenable> SkErodeImageFilter::CreateProc(SkReadBuffer& buffer) {
     SK_IMAGEFILTER_UNFLATTEN_COMMON(common, 1);
     const int width = buffer.readInt();
     const int height = buffer.readInt();
-    return Create(width, height, common.getInput(0).get(), &common.cropRect());
+    return sk_sp<SkFlattenable>(Create(width, height, common.getInput(0).get(),
+                                       &common.cropRect()));
 }
 
-SkFlattenable* SkDilateImageFilter::CreateProc(SkReadBuffer& buffer) {
+sk_sp<SkFlattenable> SkDilateImageFilter::CreateProc(SkReadBuffer& buffer) {
     SK_IMAGEFILTER_UNFLATTEN_COMMON(common, 1);
     const int width = buffer.readInt();
     const int height = buffer.readInt();
-    return Create(width, height, common.getInput(0).get(), &common.cropRect());
+    return sk_sp<SkFlattenable>(Create(width, height, common.getInput(0).get(),
+                                       &common.cropRect()));
 }
 
 #ifndef SK_IGNORE_TO_STRING