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

device/serial/data_source_sender.cc

Index: device/serial/data_source_sender.cc
diff --git a/device/serial/data_source_sender.cc b/device/serial/data_source_sender.cc
index c240303973c4dee55b400205544088446e11d29e..e67a81ea17e189fbb6961bb75438e01d91c6fcfb 100644
--- a/device/serial/data_source_sender.cc
+++ b/device/serial/data_source_sender.cc
@@ -90,9 +90,8 @@ DataSourceSender::~DataSourceSender() {
 
 void DataSourceSender::Init(mojo::ScopedDataPipeProducerHandle handle) {
   // This should never occur. |handle_| is only valid and |pending_send_| is
-  // only set after Init is called. Receiving an invalid |handle| from the
-  // client is also unrecoverable.
-  if (pending_send_ || handle_.is_valid() || !handle.is_valid() || shut_down_) {
+  // only set after Init is called.
+  if (pending_send_ || handle_.is_valid() || shut_down_) {
     DispatchFatalError();
     return;
   }