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/tools/debugger/SkDrawCommand.cpp b/tools/debugger/SkDrawCommand.cpp
index 5dc6079..af1270c 100644
--- a/tools/debugger/SkDrawCommand.cpp
+++ b/tools/debugger/SkDrawCommand.cpp
@@ -1125,8 +1125,7 @@
         Json::Value jsonShader = jsonPaint[SKDEBUGCANVAS_ATTRIBUTE_SHADER];
         SkShader* shader = (SkShader*) load_flattenable(jsonShader, urlDataManager);
         if (shader != nullptr) {
-            target->setShader(shader);
-            shader->unref();
+            target->setShader(sk_ref_sp(shader));
         }
     }
 }