Mojom: Blanket change to mark all pointer/handle fields as nullable.

device/serial/serial.mojom

Index: device/serial/serial.mojom
diff --git a/device/serial/serial.mojom b/device/serial/serial.mojom
index 23b12b374a512d98926ed54ac4edd0f540517664..a2ea53fafba5dcede33edb68bbee73c6b4cbc00f 100644
--- a/device/serial/serial.mojom
+++ b/device/serial/serial.mojom
@@ -80,22 +80,22 @@ struct DeviceControlSignals {
 };
 
 interface SerialService {
-  GetDevices() => (DeviceInfo[] devices);
+  GetDevices() => (DeviceInfo?[]? devices);
 
   // Creates a |Connection| to |path| with options specified by |options|,
   // returning it via |connection|. This will fail and |connection| will not be
   // usable if |path| does not specify a valid serial device or there is an
   // error connecting to or configuring the connection.
-  Connect(string path,
-          ConnectionOptions options,
-          Connection& connection);
+  Connect(string? path,
+          ConnectionOptions? options,
+          Connection&? connection);
 };
 
 interface Connection {
   GetInfo() => (ConnectionInfo? info);
-  SetOptions(ConnectionOptions options) => (bool success);
-  SetControlSignals(HostControlSignals signals) => (bool success);
-  GetControlSignals() => (DeviceControlSignals signals);
+  SetOptions(ConnectionOptions? options) => (bool success);
+  SetControlSignals(HostControlSignals? signals) => (bool success);
+  GetControlSignals() => (DeviceControlSignals? signals);
   Flush() => (bool success);
 };