Updates to skimage tool to use it for testing.

src/images/SkImageDecoder_libjpeg.cpp

Index: src/images/SkImageDecoder_libjpeg.cpp
diff --git a/src/images/SkImageDecoder_libjpeg.cpp b/src/images/SkImageDecoder_libjpeg.cpp
index 9920bacc45302e6c5ee3540cc62e3c0262d3683a..c291715ca9a31c745cfc2b6e971e6337f6bc5164 100644
--- a/src/images/SkImageDecoder_libjpeg.cpp
+++ b/src/images/SkImageDecoder_libjpeg.cpp
@@ -121,6 +121,7 @@ protected:
     virtual bool onDecodeRegion(SkBitmap* bitmap, const SkIRect& rect) SK_OVERRIDE;
 #endif
     virtual bool onDecode(SkStream* stream, SkBitmap* bm, Mode) SK_OVERRIDE;
+    virtual Format onGetFormat(SkStream*) const SK_OVERRIDE;
 
 private:
     SkJPEGImageIndex* fImageIndex;
@@ -993,14 +994,7 @@ protected:
     }
 };
 
-///////////////////////////////////////////////////////////////////////////////
-DEFINE_DECODER_CREATOR(JPEGImageDecoder);
-DEFINE_ENCODER_CREATOR(JPEGImageEncoder);
-///////////////////////////////////////////////////////////////////////////////
-
-#include "SkTRegistry.h"
-
-static SkImageDecoder* sk_libjpeg_dfactory(SkStream* stream) {
+static bool is_jpeg(SkStream* stream) {
     static const unsigned char gHeader[] = { 0xFF, 0xD8, 0xFF };
     static const size_t HEADER_SIZE = sizeof(gHeader);
 
@@ -1008,12 +1002,33 @@ static SkImageDecoder* sk_libjpeg_dfactory(SkStream* stream) {
     size_t len = stream->read(buffer, HEADER_SIZE);
 
     if (len != HEADER_SIZE) {
-        return NULL;   // can't read enough
+        return false;   // can't read enough
     }
     if (memcmp(buffer, gHeader, HEADER_SIZE)) {
-        return NULL;
+        return false;
+    }
+    return true;
+}
+
+SkImageDecoder::Format SkJPEGImageDecoder::onGetFormat(SkStream* stream) const {
+    if (is_jpeg(stream)) {
+        return kJPEG_Format;
+    }
+    return kUnknown_Format;
+}
+
+///////////////////////////////////////////////////////////////////////////////
+DEFINE_DECODER_CREATOR(JPEGImageDecoder);
+DEFINE_ENCODER_CREATOR(JPEGImageEncoder);
+///////////////////////////////////////////////////////////////////////////////
+
+#include "SkTRegistry.h"
+
+static SkImageDecoder* sk_libjpeg_dfactory(SkStream* stream) {
+    if (is_jpeg(stream)) {
+        return SkNEW(SkJPEGImageDecoder);
     }
-    return SkNEW(SkJPEGImageDecoder);
+    return NULL;
 }
 
 static SkImageEncoder* sk_libjpeg_efactory(SkImageEncoder::Type t) {