Update platform classes to use OVERRIDE / FINAL when needed

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 4437634f9a937be6aeb437b11e7404b49f1725b0..f309a22b4a59a13622aea22b6b91b299932349db 100644
--- a/Source/platform/image-decoders/png/PNGImageDecoder.h
+++ b/Source/platform/image-decoders/png/PNGImageDecoder.h
@@ -42,13 +42,13 @@ public:
     virtual ~PNGImageDecoder();
 
     // ImageDecoder
-    virtual String filenameExtension() const { return "png"; }
-    virtual bool isSizeAvailable();
-    virtual ImageFrame* frameBufferAtIndex(size_t);
+    virtual String filenameExtension() const OVERRIDE { return "png"; }
+    virtual bool isSizeAvailable() OVERRIDE;
+    virtual ImageFrame* frameBufferAtIndex(size_t) OVERRIDE;
     // CAUTION: setFailed() deletes |m_reader|.  Be careful to avoid
     // accessing deleted memory, especially when calling this from inside
     // PNGImageReader!
-    virtual bool setFailed();
+    virtual bool setFailed() OVERRIDE;
 
     // Callbacks from libpng
     void headerAvailable();