mojo: Allow circular dependencies between structs

mojo/public/interfaces/bindings/tests/regression_tests.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.
 
 // Module containing entities for regression tests of the generator. Entities
 // must never be modified, instead new entity must be added to add new tests.
 [JavaPackage="org.chromium.mojo.bindings.test.mojom.regression_tests"]
 module regression_tests {
 
 interface CheckMethodWithEmptyResponse {
@@ skipping 16 matching lines @@
 };
 
 enum EnumWithNumbers {
   k_2_1 = 4
 };
 
 enum EnumWithK {
   K = 0
 };
 
+struct Edge {
+  Vertex? v;
+};
+
+struct Vertex {
+  EmptyStruct? e;
+};
+
+struct EmptyStruct {
+};
+
+struct A {
+  B? b;
+};
+
+struct B {
+  A? a;
+};
+
 }  // module imported