Break up common_custom_types.mojom.

mojo/common/unguessable_token.mojom

Index: mojo/common/unguessable_token.mojom
diff --git a/mojo/common/unguessable_token.mojom b/mojo/common/unguessable_token.mojom
new file mode 100644
index 0000000000000000000000000000000000000000..32797171caf1f9dc3f7cd04b41adb8f32a9ab197
--- /dev/null
+++ b/mojo/common/unguessable_token.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.common.mojom;
+
+// Corresponds to |base::UnguessableToken| in base/unguessable_token.h
+struct UnguessableToken {
+  uint64 high;
+  uint64 low;
+};