switch surface to sk_sp

gm/imagealphathreshold.cpp

Index: gm/imagealphathreshold.cpp
diff --git a/gm/imagealphathreshold.cpp b/gm/imagealphathreshold.cpp
index eafc32f8d62706aea64d7b26b4ce9b1960c52a75..8d65cf8ee756d71cf1b51c0ff48967ed62baf054 100644
--- a/gm/imagealphathreshold.cpp
+++ b/gm/imagealphathreshold.cpp
@@ -95,9 +95,9 @@ protected:
 
     void onDraw(SkCanvas* canvas) override {
         SkImageInfo info = SkImageInfo::MakeN32(WIDTH, HEIGHT, kOpaque_SkAlphaType);
-        SkAutoTUnref<SkSurface> surface(canvas->newSurface(info));
+        auto surface(canvas->makeSurface(info));
         if (nullptr == surface) {
-            surface.reset(SkSurface::NewRaster(info));
+            surface = SkSurface::MakeRaster(info);
         }
         surface->getCanvas()->clear(SK_ColorWHITE);
         draw_rects(surface->getCanvas());