Add a partial Mojo serial connection interface and implementation.

device/serial/BUILD.gn

Index: device/serial/BUILD.gn
diff --git a/device/serial/BUILD.gn b/device/serial/BUILD.gn
index 0b38e93cbaea779b17fde1dd439fa62eb02467dd..9250c9fea39450f8b55a9f28d9f148ee95dcd935 100644
--- a/device/serial/BUILD.gn
+++ b/device/serial/BUILD.gn
@@ -9,6 +9,10 @@ static_library("serial") {
   output_name = "device_serial"
 
   sources = [
+    "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",
@@ -17,6 +21,14 @@ static_library("serial") {
     "serial_device_enumerator_mac.h",
     "serial_device_enumerator_win.cc",
     "serial_device_enumerator_win.h",
+    "serial_io_handler.cc",
+    "serial_io_handler.h",
+    "serial_io_handler_posix.cc",
+    "serial_io_handler_posix.h",
+    "serial_io_handler_win.cc",
+    "serial_io_handler_win.h",
+    "serial_service_impl.cc",
+    "serial_service_impl.h",
   ]
 
   if (is_linux) {
@@ -28,6 +40,20 @@ static_library("serial") {
   ]
 }
 
+# GYP version: device/serial/serial.gyp:device_serial_test_util
+static_library("test_util") {
+  output_name = "device_serial_test_util"
+
+  sources = [
+    "test_serial_io_handler.cc",
+    "test_serial_io_handler.h",
+  ]
+
+  deps = [
+    ":serial",
+  ]
+}
+
 mojom("serial_mojo") {
   visibility = ":serial"