Implement MouseEvent buttons attribute.

Source/core/events/WheelEvent.cpp

Index: Source/core/events/WheelEvent.cpp
diff --git a/Source/core/events/WheelEvent.cpp b/Source/core/events/WheelEvent.cpp
index 418b5d14772386fff5a649e0dff72ddf6b0f7a03..354a05cf45d8bad301c70047fc88d3698be3c4ba 100644
--- a/Source/core/events/WheelEvent.cpp
+++ b/Source/core/events/WheelEvent.cpp
@@ -60,10 +60,10 @@ WheelEvent::WheelEvent(const AtomicString& type, const WheelEventInit& initializ
 
 WheelEvent::WheelEvent(const FloatPoint& wheelTicks, const FloatPoint& rawDelta, unsigned deltaMode,
     PassRefPtrWillBeRawPtr<AbstractView> view, const IntPoint& screenLocation, const IntPoint& pageLocation,
-    bool ctrlKey, bool altKey, bool shiftKey, bool metaKey)
+    bool ctrlKey, bool altKey, bool shiftKey, bool metaKey, unsigned modifiers)
     : MouseEvent(EventTypeNames::wheel, true, true, view, 0, screenLocation.x(), screenLocation.y(),
-        pageLocation.x(), pageLocation.y(), 0, 0, ctrlKey, altKey, shiftKey, metaKey, 0, nullptr,
-        nullptr, false, PlatformMouseEvent::RealOrIndistinguishable)
+        pageLocation.x(), pageLocation.y(), 0, 0, ctrlKey, altKey, shiftKey, metaKey, 0, modifiers,
+        nullptr, nullptr, false, PlatformMouseEvent::RealOrIndistinguishable)
     , m_wheelDelta(wheelTicks.x() * TickMultiplier, wheelTicks.y() * TickMultiplier)
     , m_deltaX(-rawDelta.x())
     , m_deltaY(-rawDelta.y())
@@ -109,7 +109,7 @@ WheelEventDispatchMediator::WheelEventDispatchMediator(const PlatformWheelEvent&
 
     setEvent(WheelEvent::create(FloatPoint(event.wheelTicksX(), event.wheelTicksY()), FloatPoint(event.deltaX(), event.deltaY()),
         deltaMode(event), view, event.globalPosition(), event.position(),
-        event.ctrlKey(), event.altKey(), event.shiftKey(), event.metaKey()));
+        event.ctrlKey(), event.altKey(), event.shiftKey(), event.metaKey(), event.modifiers()));
 }
 
 WheelEvent* WheelEventDispatchMediator::event() const