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..1ec3b0a98d32da301577ca1a0dedbf046f5b1021 100644
--- a/Source/platform/PlatformMouseEvent.h
+++ b/Source/platform/PlatformMouseEvent.h
@@ -46,6 +46,7 @@ public:
     PlatformMouseEvent()
         : PlatformEvent(PlatformEvent::MouseMoved)
         , m_button(NoButton)
+        , m_buttons(0)
         , m_clickCount(0)
         , m_synthesized(RealOrIndistinguishable)
         , m_modifierFlags(0)
@@ -57,6 +58,7 @@ public:
         , m_position(position)
         , m_globalPosition(globalPosition)
         , m_button(button)
+        , m_buttons(0)
         , m_clickCount(clickCount)
         , m_synthesized(RealOrIndistinguishable)
         , m_modifierFlags(0)
@@ -68,6 +70,7 @@ public:
         , m_position(position)
         , m_globalPosition(globalPosition)
         , m_button(button)
+        , m_buttons(0)
         , m_clickCount(clickCount)
         , m_synthesized(synthesized)
         , m_modifierFlags(0)
@@ -79,6 +82,7 @@ public:
         , m_position(position)
         , m_globalPosition(globalPosition)
         , m_button(button)
+        , m_buttons(0)

[Rick Byers, 2014/11/14 19:48:09]

I think using 0 here is a bug.  You probably need to let the caller supply the
value (here and in the above overload - though maybe that overload can be
removed).  Eg. see EventHandler::handleGestureTap which creates mouse events for
a tap.  The events you create there will incorrectly report buttons=0.  Please
also add a layout test that verifies the buttons value of the mouse events
generated for a GestureTap.

[zino, 2014/11/18 14:19:35]

Good point! Thank you.

Done.

         , m_clickCount(clickCount)
         , m_synthesized(synthesized)
         , m_modifierFlags(0)
@@ -90,6 +94,7 @@ public:
     const IntPoint& movementDelta() const { return m_movementDelta; }
 
     MouseButton button() const { return m_button; }
+    unsigned short buttons() const { return m_buttons; }
     int clickCount() const { return m_clickCount; }
     unsigned modifierFlags() const { return m_modifierFlags; }
     bool fromTouch() const { return m_synthesized == FromTouch; }
@@ -100,6 +105,7 @@ protected:
     IntPoint m_globalPosition;
     IntPoint m_movementDelta;
     MouseButton m_button;
+    unsigned short m_buttons;
     int m_clickCount;
     SyntheticEventType m_synthesized;
     unsigned m_modifierFlags;