Bug 1582567. Stop emitting 'virtual' in codegen if we're already emitting 'override'. r=edgar

Differential Revision: https://phabricator.services.mozilla.com/D46527

--HG--
extra : moz-landing-system : lando

dom/bindings/Codegen.py

@@ -10834,7 +10834,7 @@ class ClassMethod(ClassItem):
         if declaring:
             if self.static:
                 decorators.append('static')
-            if self.virtual:
+            if self.virtual and not self.override:
                 decorators.append('virtual')
         if decorators:
             return ' '.join(decorators) + ' '