Break up common_custom_types.mojom.

mojo/common/string16.mojom

Index: mojo/common/string16.mojom
diff --git a/mojo/common/string16.mojom b/mojo/common/string16.mojom
new file mode 100644
index 0000000000000000000000000000000000000000..173c8670cd151ea2ee45cd66027a3306b6012365
--- /dev/null
+++ b/mojo/common/string16.mojom
@@ -0,0 +1,12 @@
+// 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::string16| in base/strings/string16.h
+// Corresponds to |WTF::String| in
+// third_party/WebKit/Source/wtf/text/WTFString.h.
+struct String16 {
+  array<uint16> data;
+};