Support buttons attribute for drag event (blink side).

Source/core/page/DragData.cpp

Index: Source/core/page/DragData.cpp
diff --git a/Source/core/page/DragData.cpp b/Source/core/page/DragData.cpp
index fbb264c22eaeb2a5eb6a4ef0999c8476278de067..fab4ec4c23cb4a03260ac3270c7ea15d133a1556 100644
--- a/Source/core/page/DragData.cpp
+++ b/Source/core/page/DragData.cpp
@@ -85,9 +85,9 @@ bool DragData::containsFiles() const
     return m_platformDragData->containsFilenames();
 }
 
-int DragData::modifierKeyState() const
+int DragData::modifiers() const
 {
-    return m_platformDragData->modifierKeyState();
+    return m_platformDragData->modifiers();
 }
 
 void DragData::asFilePaths(Vector<String>& result) const