Explicitly put serial port fd in non-blocking mode on Chrome OS.

device/serial/serial_io_handler_posix.cc

Index: device/serial/serial_io_handler_posix.cc
diff --git a/device/serial/serial_io_handler_posix.cc b/device/serial/serial_io_handler_posix.cc
index 59582c5b84e7dea1ffe4b2d8306a103e6e0baebe..8191cc21f3153fc829fa79f018f95144948a23c7 100644
--- a/device/serial/serial_io_handler_posix.cc
+++ b/device/serial/serial_io_handler_posix.cc
@@ -7,6 +7,7 @@
 #include <sys/ioctl.h>
 #include <termios.h>
 
+#include "base/files/file_util.h"
 #include "base/posix/eintr_wrapper.h"
 #include "build/build_config.h"
 
@@ -283,6 +284,15 @@ bool SerialIoHandlerPosix::ConfigurePortImpl() {
   return true;
 }
 
+bool SerialIoHandlerPosix::PostOpen() {
+#if defined(OS_CHROMEOS)
+  // The Chrome OS permission broker does not open devices in async mode.
+  return base::SetNonBlocking(file().GetPlatformFile());
+#else
+  return true;
+#endif
+}
+
 SerialIoHandlerPosix::SerialIoHandlerPosix(
     scoped_refptr<base::SingleThreadTaskRunner> file_thread_task_runner,
     scoped_refptr<base::SingleThreadTaskRunner> ui_thread_task_runner)