Add timing SkCodec to nanobench.

bench/DecodingBench.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
+#ifndef DecodingBench_DEFINED
+#define DecodingBench_DEFINED
+
 #include "Benchmark.h"
 #include "SkImageDecoder.h"
 #include "SkImageInfo.h"
 #include "SkStream.h"
 #include "SkString.h"
 
 /*
  *
  * This benchmark is designed to test the performance of image decoding.
  * It is invoked from the nanobench.cpp file.
  *
  */
 class DecodingBench : public Benchmark {
 public:
     DecodingBench(SkString path, SkColorType colorType);
 
 protected:
     const char* onGetName() override;
     bool isSuitableFor(Backend backend) override;
     void onDraw(const int n, SkCanvas* canvas) override;
     
 private:
     SkString fName;
     SkColorType fColorType;
     SkAutoTDelete<SkMemoryStream> fStream;
-    SkAutoTDelete<SkImageDecoder> fDecoder;
     typedef Benchmark INHERITED;
 };
+#endif // DecodingBench_DEFINED