Break up common_custom_types.mojom.

mojo/common/file.mojom

Index: mojo/common/file.mojom
diff --git a/mojo/common/file.mojom b/mojo/common/file.mojom
new file mode 100644
index 0000000000000000000000000000000000000000..fe224734d94b3488a2354a2156a87af94648f1c5
--- /dev/null
+++ b/mojo/common/file.mojom
@@ -0,0 +1,10 @@
+// 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::File| in base/files/file.h
+struct File {
+  handle fd;
+};