Fix gr-dropdown property definition

@property decorator was missed during JS->TS migartion

Change-Id: Iea9e3b41998c6c848cce391f8102ca49305973d0

polygerrit-ui/app/elements/shared/gr-dropdown/gr-dropdown.ts

@@ -102,6 +102,7 @@ export class GrDropdown extends KeyboardShortcutMixin(
    * Style the dropdown trigger as a link (rather than a button).
    */
 
+  @property({type: Boolean})
   link = false;
 
   @property({type: Number})