switch xfermodes over to sk_sp

waiting on https://codereview.chromium.org/1835163002/

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1832223002

Review URL: https://codereview.chromium.org/1832223002
diff --git a/tools/debugger/SkDrawCommand.cpp b/tools/debugger/SkDrawCommand.cpp
index fcd2885..a32e0b6 100644
--- a/tools/debugger/SkDrawCommand.cpp
+++ b/tools/debugger/SkDrawCommand.cpp
@@ -1169,10 +1169,9 @@
                                         SkPaint* target) {
     if (jsonPaint.isMember(SKDEBUGCANVAS_ATTRIBUTE_XFERMODE)) {
         Json::Value jsonXfermode = jsonPaint[SKDEBUGCANVAS_ATTRIBUTE_XFERMODE];
-        SkXfermode* xfermode = (SkXfermode*) load_flattenable(jsonXfermode, urlDataManager);
+        sk_sp<SkXfermode> xfermode((SkXfermode*) load_flattenable(jsonXfermode, urlDataManager));
         if (xfermode != nullptr) {
             target->setXfermode(xfermode);
-            xfermode->unref();
         }
     }
 }