Made PlatformWheelEvent a subclass of PlatformMouseEvent.

third_party/WebKit/Source/platform/PlatformMouseEvent.h

Index: third_party/WebKit/Source/platform/PlatformMouseEvent.h
diff --git a/third_party/WebKit/Source/platform/PlatformMouseEvent.h b/third_party/WebKit/Source/platform/PlatformMouseEvent.h
index 28c9056bdc42b040991c434655d775be5ad6f30e..cb193184880547d9cd773a61865395291fd61857 100644
--- a/third_party/WebKit/Source/platform/PlatformMouseEvent.h
+++ b/third_party/WebKit/Source/platform/PlatformMouseEvent.h
@@ -46,7 +46,12 @@ public:
     };
 
     PlatformMouseEvent()
-        : PlatformEvent(PlatformEvent::MouseMoved)
+        : PlatformMouseEvent(PlatformEvent::MouseMoved)
+    {
+    }
+
+    explicit PlatformMouseEvent(EventType type)
+        : PlatformEvent(type)
         , m_clickCount(0)
         , m_synthesized(RealOrIndistinguishable)
     {