Support buttons attribute for drag event (blink side).

Source/core/clipboard/DataObject.h

Index: Source/core/clipboard/DataObject.h
diff --git a/Source/core/clipboard/DataObject.h b/Source/core/clipboard/DataObject.h
index bba46a9cae57d44ea906ce0d1fe9dcac3bdbb487..123b41cd9a177a6a74f6d9e34bdef94007c5fa99 100644
--- a/Source/core/clipboard/DataObject.h
+++ b/Source/core/clipboard/DataObject.h
@@ -94,8 +94,8 @@ public:
     // Used to handle files (images) being dragged out.
     void addSharedBuffer(const String& name, PassRefPtr<SharedBuffer>);
 
-    int modifierKeyState() const { return m_modifierKeyState; }
-    void setModifierKeyState(int modifierKeyState) { m_modifierKeyState = modifierKeyState; }
+    int modifiers() const { return m_modifiers; }
+    void setModifiers(int modifiers) { m_modifiers = modifiers; }
 
     DECLARE_TRACE();
 
@@ -110,8 +110,8 @@ private:
 
     WillBeHeapVector<RefPtrWillBeMember<DataObjectItem> > m_itemList;
 
-    // State of Shift/Ctrl/Alt/Meta keys.
-    int m_modifierKeyState;
+    // State of Shift/Ctrl/Alt/Meta keys and Left/Right/Middle mouse buttons
+    int m_modifiers;
     String m_filesystemId;
 };