return and take SkImageGenerator as unique_ptr

BUG=skia:

Change-Id: I4bc11042dd1dbf1eabd40af206027bc65acc3186
Reviewed-on: https://skia-review.googlesource.com/8444
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Matt Sarett <msarett@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
diff --git a/gm/imagemasksubset.cpp b/gm/imagemasksubset.cpp
index 80e63f6..8028278 100644
--- a/gm/imagemasksubset.cpp
+++ b/gm/imagemasksubset.cpp
@@ -9,6 +9,7 @@
 #include "SkCanvas.h"
 #include "SkImage.h"
 #include "SkImageGenerator.h"
+#include "SkMakeUnique.h"
 #include "SkSurface.h"
 #include "sk_tool_utils.h"
 
@@ -59,7 +60,7 @@
 
     // SkImage_Generator
     [](SkCanvas*, const SkImageInfo& info) -> sk_sp<SkImage> {
-        return SkImage::MakeFromGenerator(new MaskGenerator(info));
+        return SkImage::MakeFromGenerator(skstd::make_unique<MaskGenerator>(info));
     },
 };