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/tools/debugger/SkDrawCommand.cpp b/tools/debugger/SkDrawCommand.cpp
index a32e0b6..d868418 100644
--- a/tools/debugger/SkDrawCommand.cpp
+++ b/tools/debugger/SkDrawCommand.cpp
@@ -726,12 +726,12 @@
     const void* data;
     int size = decode_data(jsonFlattenable[SKDEBUGCANVAS_ATTRIBUTE_DATA], urlDataManager, &data);
     SkValidatingReadBuffer buffer(data, size);
-    SkFlattenable* result = factory(buffer);
+    sk_sp<SkFlattenable> result = factory(buffer);
     if (!buffer.isValid()) {
         SkDebugf("invalid buffer loading flattenable\n");
         return nullptr;
     }
-    return result;
+    return result.release();
 }
 
 static SkColorType colortype_from_name(const char* name) {