Add timing SkCodec to nanobench.

bench/DecodingBench.cpp

Index: bench/DecodingBench.cpp
diff --git a/bench/DecodingBench.cpp b/bench/DecodingBench.cpp
index c8490ca7a89cf2d91fa8f5aa0b4866d3ee5b18cd..31796565d46cac19cda5a77ed27a6641d78f304c 100644
--- a/bench/DecodingBench.cpp
+++ b/bench/DecodingBench.cpp
@@ -41,7 +41,11 @@ DecodingBench::DecodingBench(SkString path, SkColorType colorType)
     // Perform setup for the decode
     SkAutoTUnref<SkData> encoded(SkData::NewFromFileName(path.c_str()));
     fStream.reset(new SkMemoryStream(encoded));
-    fDecoder.reset(SkImageDecoder::Factory(fStream.get()));
+#ifdef SK_DEBUG
+    // Ensure that we can create a decoder.
+    SkAutoTDelete<SkImageDecoder> decoder(SkImageDecoder::Factory(fStream.get()));
+    SkASSERT(decoder != NULL);
+#endif
 }
 
 const char* DecodingBench::onGetName() {
@@ -54,9 +58,13 @@ bool DecodingBench::isSuitableFor(Backend backend) {
 
 void DecodingBench::onDraw(const int n, SkCanvas* canvas) {
     SkBitmap bitmap;
+    SkAutoTDelete<SkImageDecoder> decoder;
+    SkAutoTDelete<SkStreamRewindable> stream;
     for (int i = 0; i < n; i++) {
-        fStream->rewind();
-        fDecoder->decode(fStream, &bitmap, fColorType,
-                         SkImageDecoder::kDecodePixels_Mode);
+        // duplicate the stream and create a new decoder to mimic the behavior of CodecBench.
+        stream.reset(fStream->duplicate());
+        decoder.reset(SkImageDecoder::Factory(stream));
+        decoder->decode(stream, &bitmap, fColorType,
+                        SkImageDecoder::kDecodePixels_Mode);
     }
 }