Support buttons attribute for drag event (blink side).

Source/core/clipboard/DataObject.cpp

Index: Source/core/clipboard/DataObject.cpp
diff --git a/Source/core/clipboard/DataObject.cpp b/Source/core/clipboard/DataObject.cpp
index 270192a5c837587c20c34659ecd6fd18c85d2fb1..df671b21788bc55ce932d29690cbc9ee43fe170a 100644
--- a/Source/core/clipboard/DataObject.cpp
+++ b/Source/core/clipboard/DataObject.cpp
@@ -220,13 +220,13 @@ void DataObject::addSharedBuffer(const String& name, PassRefPtr<SharedBuffer> bu
 }
 
 DataObject::DataObject()
-    : m_modifierKeyState(0)
+    : m_modifiers(0)
 {
 }
 
 DataObject::DataObject(const DataObject& other)
     : m_itemList(other.m_itemList)
-    , m_modifierKeyState(0)
+    , m_modifiers(0)
 {
 }