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

desktop/themes/typography.less

@@ -87,6 +87,14 @@
   color: @background-default;
 }
 
+// grow dropdown buttons like plain button
+.ant-dropdown-button {
+  flex-grow: 1;
+  & button:first-child {
+    flex-grow: 1;
+  }
+}
+
 .flipperlegacy_design {
   // Prevents ANT breaking global styles implicitly used by old Flipper design
   line-height: 1;