make sure Dropdown.Button uses flex like Button
make sure Dropdown.Button uses flex like Button

Summary: Button fills out by default in Ant design if used in a flex container. Dropdown.Button doesn't do so. With this change

Reviewed By: jknoxville

Differential Revision: D26045409

fbshipit-source-id: 593b767aa7e38a98c8887b426f36fe41cb41bfd3

facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper