Adding new benchmark to test image decoding performance.

bench/DecodingSubsetBench.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.
+ */
+
+#include "Benchmark.h"
+#include "SkImageInfo.h"
+#include "SkString.h"
+
+/*
+ *
+ * This benchmark is designed to test the performance of image decoding.
+ * It is invoked from the nanobench.cpp file.
+ *
+ */
+class DecodingSubsetBench : public Benchmark {
+public:
+    DecodingSubsetBench(SkString path, SkColorType colorType,
+            const int divisor);
+
+protected:
+    const char* onGetName() SK_OVERRIDE;
+    bool isSuitableFor(Backend backend) SK_OVERRIDE;
+    void onDraw(const int n, SkCanvas* canvas) SK_OVERRIDE;
+    
+private:
+    SkString fName;
+    SkString fPath;
+    SkColorType fColorType;
+    const int fDivisor;
+    typedef Benchmark INHERITED;
+};