mojo bindings: Support enums with no values

mojo/public/interfaces/bindings/tests/validation_test_interfaces.mojom

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 
 [JavaPackage="org.chromium.mojo.bindings.test.mojom.mojo"]
 module mojo.test;
 
 struct StructA {
   uint64 i;
@@ skipping 38 matching lines @@
   ENUM_A_1
 };
 
 [Extensible]
 enum EnumB {
   ENUM_B_0,
   ENUM_B_1,
   ENUM_B_2
 };
 
+// A non-extensible enum with no values is valid, but about as useless as
+// you would expect: it will fail validation for all values.
+enum EmptyEnum {};
+
+[Extensible]
+enum ExtensibleEmptyEnum {};
+
 union UnionA {
   StructA struct_a;
   bool b;
 };
 
 // This interface is used for testing bounds-checking in the mojom
 // binding code. If you add a method please update the files
 // ./data/validation/boundscheck_*. If you add a response please update
 // ./data/validation/resp_boundscheck_*.
 interface BoundsCheckTestInterface {
@@ skipping 16 matching lines @@
   Method11(StructG param0);
   Method12(float param0) => (float param0);
   Method13(InterfaceA? param0, uint32 param1, InterfaceA? param2);
   Method14(EnumA param0, EnumB param1);
   Method15(array<EnumA>? param0, array<EnumB>? param1);
   Method16(map<EnumA, EnumA>? param0);
   Method17(array<InterfaceA> param0);
   Method18(UnionA? param0);
   Method19(Recursive recursive);
   Method20(map<StructB, uint8> param0);
+  Method21(ExtensibleEmptyEnum param0);
+  Method22(EmptyEnum param0);
 };
 
 struct BasicStruct {
   int32 a;
 };
 
 interface IntegrationTestInterface {
   Method0(BasicStruct param0) => (array<uint8> param0);
 };
 
@@ skipping 12 matching lines @@
 struct StructWithEnum {
   enum EnumWithin {
     A, B, C, D
   };
 };
 
 // This is used to test that deeply recursive structures don't blow the stack.
 struct Recursive {
   Recursive? recursive;
 };