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 1273aaa5857fe658974cfa354749e62f27c9ce53..198733028dc9a320479875bb6d23c21a1f9fa941 100644
--- a/public/web/WebView.h
+++ b/public/web/WebView.h
@@ -343,15 +343,15 @@ 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;