[serial] When a read is requested on posix platforms, try the read immediately.

device/serial/serial_io_handler_posix.h

Index: device/serial/serial_io_handler_posix.h
diff --git a/device/serial/serial_io_handler_posix.h b/device/serial/serial_io_handler_posix.h
index d03d9c500119c7b78501eabfd1fc8cb251eb37cb..28f10c434a7135ec14d4b1ea1e1d3dca6df1e90d 100644
--- a/device/serial/serial_io_handler_posix.h
+++ b/device/serial/serial_io_handler_posix.h
@@ -49,6 +49,11 @@ class SerialIoHandlerPosix : public SerialIoHandler,
       scoped_refptr<base::SingleThreadTaskRunner> ui_thread_task_runner);
   ~SerialIoHandlerPosix() override;
 
+  void AttemptRead(bool within_read);
+  void RunReadCompleted(bool within_read,
+                        int bytes_read,
+                        serial::ReceiveError error);
+
   // base::MessageLoopForIO::Watcher implementation.
   void OnFileCanWriteWithoutBlocking(int fd) override;
   void OnFileCanReadWithoutBlocking(int fd) override;