Support buttons attribute for drag event (blink side).

public/web/WebView.h

Index: public/web/WebView.h
diff --git a/public/web/WebView.h b/public/web/WebView.h
index 774d117bf2106323be309e1e96c71587a3bf1574..526962b6c06611bfcc80f152439290e9d47e004c 100644
--- a/public/web/WebView.h
+++ b/public/web/WebView.h
@@ -345,15 +345,15 @@ public:
         const WebDragData&,
         const WebPoint& clientPoint, const WebPoint& screenPoint,
         WebDragOperationsMask operationsAllowed,
-        int keyModifiers) = 0;
+        int modifiers) = 0;
     virtual WebDragOperation dragTargetDragOver(
         const WebPoint& clientPoint, const WebPoint& screenPoint,
         WebDragOperationsMask operationsAllowed,
-        int keyModifiers) = 0;
+        int modifiers) = 0;
     virtual void dragTargetDragLeave() = 0;
     virtual void dragTargetDrop(
         const WebPoint& clientPoint, const WebPoint& screenPoint,
-        int keyModifiers) = 0;
+        int modifiers) = 0;
 
     // Retrieves a list of spelling markers.
     virtual void spellingMarkers(WebVector<uint32_t>* markers) = 0;