Add WebPointerProperties type.

public/web/WebInputEvent.h

Index: public/web/WebInputEvent.h
diff --git a/public/web/WebInputEvent.h b/public/web/WebInputEvent.h
index b5d0a8bc6052b1f56fabe2537e52a02a86eb92b8..f9fcb5497091c2bd46c3c38165184b800669eff1 100644
--- a/public/web/WebInputEvent.h
+++ b/public/web/WebInputEvent.h
@@ -33,6 +33,7 @@
 
 #include "../platform/WebCommon.h"
 #include "../platform/WebGestureDevice.h"
+#include "../platform/WebPointerProperties.h"
 #include "../platform/WebRect.h"
 #include "WebTouchPoint.h"
 
@@ -300,16 +301,8 @@ public:
 
 // WebMouseEvent --------------------------------------------------------------
 
-class WebMouseEvent : public WebInputEvent {
+class WebMouseEvent : public WebInputEvent, public WebPointerProperties {
 public:
-    enum Button {
-        ButtonNone = -1,
-        ButtonLeft,
-        ButtonMiddle,
-        ButtonRight
-    };
-
-    Button button;
     int x;
     int y;
     int windowX;
@@ -322,7 +315,7 @@ public:
 
     WebMouseEvent()
         : WebInputEvent(sizeof(WebMouseEvent))
-        , button(ButtonNone)
+        , WebPointerProperties()
         , x(0)
         , y(0)
         , windowX(0)
@@ -338,7 +331,7 @@ public:
 protected:
     explicit WebMouseEvent(unsigned sizeParam)
         : WebInputEvent(sizeParam)
-        , button(ButtonNone)
+        , WebPointerProperties()
         , x(0)
         , y(0)
         , windowX(0)
@@ -522,6 +515,7 @@ public:
 
 // WebTouchEvent --------------------------------------------------------------
 
+// TODO(e_hakkinen): Replace with WebPointerEvent. crbug.com/508283
 class WebTouchEvent : public WebInputEvent {
 public:
     // Maximum number of simultaneous touches supported on