Implement break signal for serial api.

extensions/browser/api/serial/serial_connection.h

Index: extensions/browser/api/serial/serial_connection.h
diff --git a/extensions/browser/api/serial/serial_connection.h b/extensions/browser/api/serial/serial_connection.h
index 7fc553590007e825dabf9d9a6092784f55d7c837..0537c3867727b893ca41308b7e00f1fc4b2887f4 100644
--- a/extensions/browser/api/serial/serial_connection.h
+++ b/extensions/browser/api/serial/serial_connection.h
@@ -112,6 +112,12 @@ class SerialConnection : public ApiResource,
   bool SetControlSignals(
       const core_api::serial::HostControlSignals& control_signals);
 
+  // Suspend character transmission. Known as setting/sending 'Break' signal.
+  bool SetBreak();
+
+  // Restore character transmission. Known as clear/stop sending 'Break' signal.
+  bool ClearBreak();
+
   // Overrides |io_handler_| for testing.
   void SetIoHandlerForTest(scoped_refptr<device::SerialIoHandler> handler);