Add options to let mojom targets override their component deps.

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

 // Copyright 2016 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 mojo.test.test_export;
 
 struct StringPair {
   string s1;
   string s2;
 };
 
 // This is a regression test. On Windows, if we export the generated class *and*
 // not explicitly disallow copy constructor and assign operator, compilation
 // will fail because it tries to use copy constructor of
 // InlinedStructPtr<StringPair>.
 struct StringPairContainer {
   array<StringPair> pairs;
 };
+
+interface ExportedInterface {};