Make some fields non-nullable in serial.mojom and data_stream.mojom.

device/serial/data_sink_receiver.cc

Index: device/serial/data_sink_receiver.cc
diff --git a/device/serial/data_sink_receiver.cc b/device/serial/data_sink_receiver.cc
index 7e3834e4c504f6698494d27da0203d534c4c23d4..cadc9e3de6e6e4aef52bf1b23793902f1979adf2 100644
--- a/device/serial/data_sink_receiver.cc
+++ b/device/serial/data_sink_receiver.cc
@@ -89,7 +89,7 @@ DataSinkReceiver::~DataSinkReceiver() {
 }
 
 void DataSinkReceiver::Init(mojo::ScopedDataPipeConsumerHandle handle) {
-  if (!handle.is_valid() || handle_.is_valid()) {
+  if (handle_.is_valid()) {
     DispatchFatalError();
     return;
   }