Implement MouseEvent buttons attribute.

Source/platform/PlatformMouseEvent.h

Index: Source/platform/PlatformMouseEvent.h
diff --git a/Source/platform/PlatformMouseEvent.h b/Source/platform/PlatformMouseEvent.h
index 488bd0c211c910e5137fe452603608f3484de634..070ab9839f6bc3030ef4bda0677b863cfb8ec647 100644
--- a/Source/platform/PlatformMouseEvent.h
+++ b/Source/platform/PlatformMouseEvent.h
@@ -61,6 +61,7 @@ public:
         , m_synthesized(RealOrIndistinguishable)
         , m_modifierFlags(0)
     {
+        addModifierFromButton(button);
     }
 
     PlatformMouseEvent(const IntPoint& position, const IntPoint& globalPosition, MouseButton button, PlatformEvent::Type type, int clickCount, Modifiers modifiers, SyntheticEventType synthesized, double timestamp)
@@ -72,6 +73,7 @@ public:
         , m_synthesized(synthesized)
         , m_modifierFlags(0)
     {
+        addModifierFromButton(button);
     }
 
     PlatformMouseEvent(const IntPoint& position, const IntPoint& globalPosition, MouseButton button, PlatformEvent::Type type, int clickCount, bool shiftKey, bool ctrlKey, bool altKey, bool metaKey, SyntheticEventType synthesized, double timestamp)
@@ -83,6 +85,7 @@ public:
         , m_synthesized(synthesized)
         , m_modifierFlags(0)
     {
+        addModifierFromButton(button);
     }
 
     const IntPoint& position() const { return m_position; }
@@ -95,6 +98,18 @@ public:
     bool fromTouch() const { return m_synthesized == FromTouch; }
     SyntheticEventType syntheticEventType() const { return m_synthesized; }
 
+private:
+    void addModifierFromButton(MouseButton button)
+    {
+        if (!fromTouch() || button == NoButton)
+            return;
+
+        const unsigned map[] = {
+            LeftButtonDown, MiddleButtonDown, RightButtonDown
+        };
+        m_modifiers |= map[button];
+    }
+
 protected:
     IntPoint m_position;
     IntPoint m_globalPosition;