Add a partial Mojo serial connection interface and implementation.

extensions/browser/api/serial/serial_connection.cc

Index: extensions/browser/api/serial/serial_connection.cc
diff --git a/extensions/browser/api/serial/serial_connection.cc b/extensions/browser/api/serial/serial_connection.cc
index 9b995866656ed890a10c903e2dab710a2a2c9e58..3653597affea6a179cf88cde4361e6b4fe69f965 100644
--- a/extensions/browser/api/serial/serial_connection.cc
+++ b/extensions/browser/api/serial/serial_connection.cc
@@ -155,13 +155,13 @@ SerialConnection::SerialConnection(const std::string& port,
       receive_timeout_(0),
       send_timeout_(0),
       paused_(false),
-      io_handler_(device::SerialIoHandler::Create()) {
+      io_handler_(device::SerialIoHandler::Create(
+          content::BrowserThread::GetMessageLoopProxyForThread(
+              content::BrowserThread::FILE))) {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
   io_handler_->Initialize(
       base::Bind(&SerialConnection::OnAsyncReadComplete, AsWeakPtr()),
-      base::Bind(&SerialConnection::OnAsyncWriteComplete, AsWeakPtr()),
-      content::BrowserThread::GetMessageLoopProxyForThread(
-          content::BrowserThread::FILE));
+      base::Bind(&SerialConnection::OnAsyncWriteComplete, AsWeakPtr()));
 }
 
 SerialConnection::~SerialConnection() {
@@ -252,9 +252,7 @@ void SerialConnection::SetIoHandlerForTest(
   io_handler_ = handler;
   io_handler_->Initialize(
       base::Bind(&SerialConnection::OnAsyncReadComplete, AsWeakPtr()),
-      base::Bind(&SerialConnection::OnAsyncWriteComplete, AsWeakPtr()),
-      content::BrowserThread::GetMessageLoopProxyForThread(
-          content::BrowserThread::FILE));
+      base::Bind(&SerialConnection::OnAsyncWriteComplete, AsWeakPtr()));
 }
 
 bool SerialConnection::GetInfo(core_api::serial::ConnectionInfo* info) const {