Add a partial Mojo serial connection interface and implementation.

device/serial/serial.gyp

Index: device/serial/serial.gyp
diff --git a/device/serial/serial.gyp b/device/serial/serial.gyp
index 5c0e386e6c069d1e2bc95be54704cf99522b60f9..dec3102ab2eaac3b9a7f956f68e30ae33bc6fc0c 100644
--- a/device/serial/serial.gyp
+++ b/device/serial/serial.gyp
@@ -36,6 +36,10 @@
       ],
       'sources': [
         'serial.mojom',
+        'serial_connection.cc',
+        'serial_connection.h',
+        'serial_connection_factory.cc',
+        'serial_connection_factory.h',
         'serial_device_enumerator.cc',
         'serial_device_enumerator.h',
         'serial_device_enumerator_linux.cc',
@@ -54,5 +58,17 @@
         'serial_service_impl.h',
       ],
     },
+    {
+      # GN version: //device/serial:test_util
+      'target_name': 'device_serial_test_util',
+      'type': 'static_library',
+      'dependencies': [
+        'device_serial',
+      ],
+      'sources': [
+        'test_serial_io_handler.cc',
+        'test_serial_io_handler.h',
+      ],
+    },
   ],
 }