Implement break signal for serial api.

device/serial/test_serial_io_handler.cc

Index: device/serial/test_serial_io_handler.cc
diff --git a/device/serial/test_serial_io_handler.cc b/device/serial/test_serial_io_handler.cc
index 9713c070f98bd1f44fc6a737eb1c42d941f48d88..0dcb048a50879dd8cea5060e871679e5d205c3fd 100644
--- a/device/serial/test_serial_io_handler.cc
+++ b/device/serial/test_serial_io_handler.cc
@@ -101,6 +101,14 @@ bool TestSerialIoHandler::SetControlSignals(
   return true;
 }
 
+bool TestSerialIoHandler::SetBreak() {
+  return true;
+}
+
+bool TestSerialIoHandler::ClearBreak() {
+  return true;
+}
+
 TestSerialIoHandler::~TestSerialIoHandler() {
 }