Add missing virtual keyword for methods with OVERRIDE

Source/platform/graphics/test/MockDiscardablePixelRef.h

Index: Source/platform/graphics/test/MockDiscardablePixelRef.h
diff --git a/Source/platform/graphics/test/MockDiscardablePixelRef.h b/Source/platform/graphics/test/MockDiscardablePixelRef.h
index 9c013d7065be40b94b43bae13566fdea190225ca..3370522f5d4f8eb364c3f085f3d7639d7de6809e 100644
--- a/Source/platform/graphics/test/MockDiscardablePixelRef.h
+++ b/Source/platform/graphics/test/MockDiscardablePixelRef.h
@@ -49,7 +49,7 @@ public:
 
     class Allocator : public SkBitmap::Allocator {
     public:
-        bool allocPixelRef(SkBitmap* dst, SkColorTable* ct) SK_OVERRIDE {
+        virtual bool allocPixelRef(SkBitmap* dst, SkColorTable* ct) SK_OVERRIDE {
             SkImageInfo info;
             if (!dst->asImageInfo(&info)) {
                 return false;