switch surface to sk_sp

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1817383002
CQ_EXTRA_TRYBOTS=client.skia.compile:Build-Ubuntu-GCC-x86_64-Release-CMake-Trybot,Build-Mac-Clang-x86_64-Release-CMake-Trybot

Review URL: https://codereview.chromium.org/1817383002
diff --git a/gm/dftext.cpp b/gm/dftext.cpp
index 545911c..0d5b1ff 100644
--- a/gm/dftext.cpp
+++ b/gm/dftext.cpp
@@ -51,9 +51,8 @@
         SkImageInfo info = SkImageInfo::MakeN32Premul(onISize());
         SkSurfaceProps props(SkSurfaceProps::kUseDeviceIndependentFonts_Flag,
                              SkSurfaceProps::kLegacyFontHost_InitType);
-        SkAutoTUnref<SkSurface> surface(SkSurface::NewRenderTarget(ctx, SkBudgeted::kNo,
-                                                                   info, 0, &props));
-        SkCanvas* canvas = surface.get() ? surface->getCanvas() : inputCanvas;
+        auto surface(SkSurface::MakeRenderTarget(ctx, SkBudgeted::kNo, info, 0, &props));
+        SkCanvas* canvas = surface ? surface->getCanvas() : inputCanvas;
         // init our new canvas with the old canvas's matrix
         canvas->setMatrix(inputCanvas->getTotalMatrix());
 #else