Base SkAutoTUnref on skstd::unique_ptr.

To further consolidate the various unique owning classes, this bases
SkAutoTUnref on skstd::unique_ptr. Users are updated because of two
breaking changes, swap now takes a reference and reset no longer
returns its argument.

Review URL: https://codereview.chromium.org/1370803002
diff --git a/tools/PictureRenderer.cpp b/tools/PictureRenderer.cpp
index 21bb933..19ee7b5 100644
--- a/tools/PictureRenderer.cpp
+++ b/tools/PictureRenderer.cpp
@@ -76,7 +76,7 @@
         return;
     }
 
-    fPicture.reset(pict)->ref();
+    fPicture.reset(SkRef(pict));
     fCanvas.reset(this->setupCanvas());
 }
 
@@ -503,7 +503,7 @@
 
     // Do not call INHERITED::init(), which would create a (potentially large) canvas which is not
     // used by bench_pictures.
-    fPicture.reset(pict)->ref();
+    fPicture.reset(SkRef(pict));
     this->CopyString(&fWritePath, writePath);
     this->CopyString(&fMismatchPath, mismatchPath);
     this->CopyString(&fInputFilename, inputFilename);