Add struct_traits and typemap for blink::WebGamepad

device/BUILD.gn

Index: device/BUILD.gn
diff --git a/device/BUILD.gn b/device/BUILD.gn
index b8f42508655e07a3bdd0e412522e4825822eb0a7..6ed38c003fbd17212d0ed411ac4e3da69f87cb77 100644
--- a/device/BUILD.gn
+++ b/device/BUILD.gn
@@ -62,6 +62,7 @@ test("device_unittests") {
     "bluetooth/test/test_bluetooth_local_gatt_service_delegate.cc",
     "bluetooth/test/test_bluetooth_local_gatt_service_delegate.h",
     "gamepad/gamepad_provider_unittest.cc",
+    "gamepad/public/interfaces/gamepad_struct_traits_unittest.cc",
     "generic_sensor/platform_sensor_and_provider_unittest_win.cc",
     "generic_sensor/platform_sensor_provider_unittest.cc",
     "power_monitor/power_monitor_message_broadcaster_unittest.cc",
@@ -81,6 +82,8 @@ test("device_unittests") {
     "//device/bluetooth:mojo",
     "//device/gamepad",
     "//device/gamepad:test_helpers",
+    "//device/gamepad/public/interfaces",
+    "//device/gamepad/public/interfaces:gamepad_struct_traits_test",
     "//device/generic_sensor",
     "//device/generic_sensor:testing",
     "//device/geolocation:unittests",
@@ -95,6 +98,7 @@ test("device_unittests") {
     "//net",
     "//testing/gmock",
     "//testing/gtest",
+    "//third_party/WebKit/public:blink_headers",
     "//tools/usb_gadget",
     "//url",
   ]