Allow Mojo structs as map keys

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 73 matching lines @@
   Method10(map<string, uint8> param0);
   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);
 };
 
 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;
 };