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

device/serial/data_stream.mojom

Index: device/serial/data_stream.mojom
diff --git a/device/serial/data_stream.mojom b/device/serial/data_stream.mojom
index 689b38148411ede1b7ca63c39c3a2f47a841027b..ae64c4ee85c316649a2ac10202729565a76e5a92 100644
--- a/device/serial/data_stream.mojom
+++ b/device/serial/data_stream.mojom
@@ -8,7 +8,7 @@ module device.serial {
 interface DataSource {
   // Initializes this DataSource with a data pipe handle to use for data
   // transmission.
-  Init(handle<data_pipe_producer> producer_handle);
+  Init(handle<data_pipe_producer>? producer_handle);
 
   // Resumes sending data after it has been stopped due to an error.
   Resume();
@@ -25,7 +25,7 @@ interface DataSourceClient {
 interface DataSink {
   // Initializes this DataSink with a data pipe handle to use for data
   // transmission.
-  Init(handle<data_pipe_consumer> consumer_handle);
+  Init(handle<data_pipe_consumer>? consumer_handle);
 
   // Requests the cancellation of any data that has been written to the pipe,
   // but has not yet been sent to the sink.