return pictures as sk_sp

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1811703002

Review URL: https://codereview.chromium.org/1811703002
diff --git a/gm/picture.cpp b/gm/picture.cpp
index 1f8ded0..904046d 100644
--- a/gm/picture.cpp
+++ b/gm/picture.cpp
@@ -9,7 +9,7 @@
 #include "SkPaint.h"
 #include "SkPictureRecorder.h"
 
-static SkPicture* make_picture() {
+static sk_sp<SkPicture> make_picture() {
     SkPictureRecorder rec;
     SkCanvas* canvas = rec.beginRecording(100, 100);
 
@@ -32,7 +32,7 @@
     paint.setXfermodeMode(SkXfermode::kPlus_Mode);
     canvas->drawRect(SkRect::MakeXYWH(25, 25, 50, 50), paint);
 
-    return rec.endRecording();
+    return rec.finishRecordingAsPicture();
 }
 
 // Exercise the optional arguments to drawPicture
@@ -45,7 +45,7 @@
 
 protected:
     void onOnceBeforeDraw() override {
-         fPicture.reset(make_picture());
+         fPicture = make_picture();
     }
 
     SkString onShortName() override {
@@ -76,7 +76,7 @@
     }
 
 private:
-    SkAutoTUnref<SkPicture> fPicture;
+    sk_sp<SkPicture> fPicture;
 
     typedef skiagm::GM INHERITED;
 };