Updates to skimage tool to use it for testing.

src/images/SkImageDecoder_libpng.cpp

Index: src/images/SkImageDecoder_libpng.cpp
diff --git a/src/images/SkImageDecoder_libpng.cpp b/src/images/SkImageDecoder_libpng.cpp
index efa4595545709a7b5de1063b6d5a66b23609f38a..aed49cf6ecb08e0d4d2ecd33ae2fac784ac82404 100644
--- a/src/images/SkImageDecoder_libpng.cpp
+++ b/src/images/SkImageDecoder_libpng.cpp
@@ -47,6 +47,9 @@ public:
     virtual Format getFormat() const SK_OVERRIDE {
         return kPNG_Format;
     }
+
+    virtual Format onGetFormat(SkStream*) const SK_OVERRIDE;
+
     virtual ~SkPNGImageDecoder() {
         SkDELETE(fImageIndex);
     }
@@ -1079,6 +1082,22 @@ bool SkPNGImageEncoder::doEncode(SkWStream* stream, const SkBitmap& bitmap,
     return true;
 }
 
+static bool is_png(SkStream* stream) {
+    char buf[PNG_BYTES_TO_CHECK];
+    if (stream->read(buf, PNG_BYTES_TO_CHECK) == PNG_BYTES_TO_CHECK &&
+        !png_sig_cmp((png_bytep) buf, (png_size_t)0, PNG_BYTES_TO_CHECK)) {
+        return true;
+    }
+    return false;
+}
+
+SkImageDecoder::Format SkPNGImageDecoder::onGetFormat(SkStream* stream) const {
+    if (is_png(stream)) {
+        return kPNG_Format;
+    }
+    return kUnknown_Format;
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 DEFINE_DECODER_CREATOR(PNGImageDecoder);
 DEFINE_ENCODER_CREATOR(PNGImageEncoder);
@@ -1087,9 +1106,7 @@ DEFINE_ENCODER_CREATOR(PNGImageEncoder);
 #include "SkTRegistry.h"
 
 SkImageDecoder* sk_libpng_dfactory(SkStream* stream) {
-    char buf[PNG_BYTES_TO_CHECK];
-    if (stream->read(buf, PNG_BYTES_TO_CHECK) == PNG_BYTES_TO_CHECK &&
-        !png_sig_cmp((png_bytep) buf, (png_size_t)0, PNG_BYTES_TO_CHECK)) {
+    if (is_png(stream)) {
         return SkNEW(SkPNGImageDecoder);
     }
     return NULL;