Add options to let mojom targets override their component deps.

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

Index: mojo/public/interfaces/bindings/tests/test_import.mojom
diff --git a/mojo/public/interfaces/bindings/tests/test_import.mojom b/mojo/public/interfaces/bindings/tests/test_import.mojom
new file mode 100644
index 0000000000000000000000000000000000000000..42014c6809eb42079537f0e8af87a9e45d5b461c
--- /dev/null
+++ b/mojo/public/interfaces/bindings/tests/test_import.mojom
@@ -0,0 +1,11 @@
+// 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_import;
+
+import "mojo/public/interfaces/bindings/tests/test_export.mojom";
+
+struct ImportingStruct {
+  mojo.test.test_export.StringPair strings;
+};