Fix virtual/override/final usage in Source/platform/.

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

 /*
  * Copyright (C) 2006 Apple Computer, Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 20 matching lines @@
 #include "wtf/OwnPtr.h"
 
 namespace blink {
 
 class PNGImageReader;
 
 class PLATFORM_EXPORT PNGImageDecoder : public ImageDecoder {
     WTF_MAKE_NONCOPYABLE(PNGImageDecoder);
 public:
     PNGImageDecoder(ImageSource::AlphaOption, ImageSource::GammaAndColorProfileOption, size_t maxDecodedBytes);
-    virtual ~PNGImageDecoder();
+    ~PNGImageDecoder() override;
 
     // ImageDecoder:
-    virtual String filenameExtension() const override { return "png"; }
-    virtual bool hasColorProfile() const override { return m_hasColorProfile; }
+    String filenameExtension() const override { return "png"; }
+    bool hasColorProfile() const override { return m_hasColorProfile; }
 
     // Callbacks from libpng
     void headerAvailable();
     void rowAvailable(unsigned char* row, unsigned rowIndex, int);
     void complete();
 
 private:
     // ImageDecoder:
-    virtual void decodeSize() override { decode(true); }
-    virtual void decode(size_t) override { decode(false); }
+    void decodeSize() override { decode(true); }
+    void decode(size_t) override { decode(false); }
 
     // Decodes the image.  If |onlySize| is true, stops decoding after
     // calculating the image size.  If decoding fails but there is no more
     // data coming, sets the "decode failure" flag.
     void decode(bool onlySize);
 
     OwnPtr<PNGImageReader> m_reader;
     bool m_hasColorProfile;
 };
 
 } // namespace blink
 
 #endif