Mojom cpp bindings: Enable non-nullable check.

device/serial/serial.mojom

Index: device/serial/serial.mojom
diff --git a/device/serial/serial.mojom b/device/serial/serial.mojom
index 8043063839f7aedc12b6749402c0cd5e24c7f770..23b12b374a512d98926ed54ac4edd0f540517664 100644
--- a/device/serial/serial.mojom
+++ b/device/serial/serial.mojom
@@ -5,12 +5,12 @@
 module device.serial {
 
 struct DeviceInfo {
-  string path;
+  string? path;
   uint16 vendor_id;
   bool has_vendor_id = false;
   uint16 product_id;
   bool has_product_id = false;
-  string display_name;
+  string? display_name;
 };
 
 enum SendError {
@@ -92,7 +92,7 @@ interface SerialService {
 };
 
 interface Connection {
-  GetInfo() => (ConnectionInfo info);
+  GetInfo() => (ConnectionInfo? info);
   SetOptions(ConnectionOptions options) => (bool success);
   SetControlSignals(HostControlSignals signals) => (bool success);
   GetControlSignals() => (DeviceControlSignals signals);