Add struct_traits and typemap for blink::WebGamepad

device/gamepad/public/interfaces/BUILD.gn

Index: device/gamepad/public/interfaces/BUILD.gn
diff --git a/device/gamepad/public/interfaces/BUILD.gn b/device/gamepad/public/interfaces/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..ffbf359a52ea3c71dfbdedb0f415a048f5abead4
--- /dev/null
+++ b/device/gamepad/public/interfaces/BUILD.gn
@@ -0,0 +1,21 @@
+# 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.
+
+import("//mojo/public/tools/bindings/mojom.gni")
+
+mojom("interfaces") {
+  sources = [
+    "gamepad.mojom",
+  ]
+}
+
+mojom("gamepad_struct_traits_test") {
+  sources = [
+    "gamepad_struct_traits_test.mojom",
+  ]
+
+  public_deps = [
+    ":interfaces",
+  ]
+}