Record UMA stats for Blink decoded image types

Source/platform/image-decoders/png/PNGImageDecoder.h

Index: Source/platform/image-decoders/png/PNGImageDecoder.h
diff --git a/Source/platform/image-decoders/png/PNGImageDecoder.h b/Source/platform/image-decoders/png/PNGImageDecoder.h
index 7f54c2e360725981b96c9c96ce4cb6b331a77995..6cb498e217ea6533cfcc61a7fa4b912b777d1cf2 100644
--- a/Source/platform/image-decoders/png/PNGImageDecoder.h
+++ b/Source/platform/image-decoders/png/PNGImageDecoder.h
@@ -51,7 +51,11 @@ public:
 
 private:
     // ImageDecoder:
-    void decodeSize() override { decode(true); }
+    void decodeSize() override
+    {
+        decode(true);
+        reportStats(ImagePNG);
+    }
     void decode(size_t) override { decode(false); }
 
     // Decodes the image.  If |onlySize| is true, stops decoding after