Add timing SkCodec to nanobench.

bench/DecodingBench.h

Index: bench/DecodingBench.h
diff --git a/bench/DecodingBench.h b/bench/DecodingBench.h
index 80a474df323abafa2568b82443611ce7d3ff59ec..65fb31283a9e08c6de6cfa450a1d8e7802321bad 100644
--- a/bench/DecodingBench.h
+++ b/bench/DecodingBench.h
@@ -5,7 +5,11 @@
  * found in the LICENSE file.
  */
 
+#ifndef DecodingBench_DEFINED
+#define DecodingBench_DEFINED
+
 #include "Benchmark.h"
+#include "SkBitmap.h"
 #include "SkImageDecoder.h"
 #include "SkImageInfo.h"
 #include "SkStream.h"
@@ -25,11 +29,13 @@ protected:
     const char* onGetName() override;
     bool isSuitableFor(Backend backend) override;
     void onDraw(const int n, SkCanvas* canvas) override;
-    
+    void onPreDraw() override;
+
 private:
-    SkString fName;
-    SkColorType fColorType;
-    SkAutoTDelete<SkMemoryStream> fStream;
-    SkAutoTDelete<SkImageDecoder> fDecoder;
+    SkString                        fName;
+    SkColorType                     fColorType;
+    SkAutoTDelete<SkMemoryStream>   fStream;
+    SkBitmap                        fBitmap;
     typedef Benchmark INHERITED;
 };
+#endif // DecodingBench_DEFINED