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

Source/platform/graphics/gpu/DrawingBuffer.h

Index: Source/platform/graphics/gpu/DrawingBuffer.h
diff --git a/Source/platform/graphics/gpu/DrawingBuffer.h b/Source/platform/graphics/gpu/DrawingBuffer.h
index 8c9fda036ee192f9e3f491c25578b70e22725b67..7c8cfad22a78d00a96b987bd31322a45124767e3 100644
--- a/Source/platform/graphics/gpu/DrawingBuffer.h
+++ b/Source/platform/graphics/gpu/DrawingBuffer.h
@@ -95,7 +95,7 @@ public:
     static PassRefPtr<DrawingBuffer> create(PassOwnPtr<WebGraphicsContext3D>, const IntSize&, PreserveDrawingBuffer, WebGraphicsContext3D::Attributes requestedAttributes);
     static void forceNextDrawingBufferCreationToFail();
 
-    virtual ~DrawingBuffer();
+    ~DrawingBuffer() override;
 
     // Destruction will be completed after all mailboxes are released.
     void beginDestruction();
@@ -167,8 +167,8 @@ public:
     WebGraphicsContext3D::Attributes getActualAttributes() const { return m_actualAttributes; }
 
     // WebExternalTextureLayerClient implementation.
-    virtual bool prepareMailbox(WebExternalTextureMailbox*, WebExternalBitmap*) override;
-    virtual void mailboxReleased(const WebExternalTextureMailbox&, bool lostResource = false) override;
+    bool prepareMailbox(WebExternalTextureMailbox*, WebExternalBitmap*) override;
+    void mailboxReleased(const WebExternalTextureMailbox&, bool lostResource = false) override;
 
     // Destroys the TEXTURE_2D binding for the owned context
     bool copyToPlatformTexture(WebGraphicsContext3D*, Platform3DObject texture, GLenum internalFormat,