[mojo-edk] Support transferring null Mach ports.

mojo/edk/system/mach_port_relay.cc

Index: mojo/edk/system/mach_port_relay.cc
diff --git a/mojo/edk/system/mach_port_relay.cc b/mojo/edk/system/mach_port_relay.cc
index 2dd40e51aae207c36ebd8c8fd19c4c5a6c301036..22beb652210ecbf0fb3b5f377509de288fc4ff6b 100644
--- a/mojo/edk/system/mach_port_relay.cc
+++ b/mojo/edk/system/mach_port_relay.cc
@@ -27,6 +27,11 @@ bool MachPortRelay::ReceivePorts(PlatformHandleVector* handles) {
     if (handle->type != PlatformHandle::Type::MACH_NAME)
       continue;
 
+    if (handle->port == MACH_PORT_NULL) {
+      handle->type = PlatformHandle::Type::MACH;
+      continue;
+    }
+
     base::mac::ScopedMachReceiveRight message_port(handle->port);
     base::mac::ScopedMachSendRight received_port(
         base::ReceiveMachPort(message_port.get()));
@@ -72,8 +77,13 @@ bool MachPortRelay::SendPortsToProcess(Channel::Message* message,
     if (handle->type != PlatformHandle::Type::MACH)
       continue;
 
+    if (handle->port == MACH_PORT_NULL) {
+      handle->type = PlatformHandle::Type::MACH_NAME;
+      num_sent++;
+      continue;
+    }
+
     mach_port_name_t intermediate_port;
-    DCHECK(handle->port != MACH_PORT_NULL);
     intermediate_port = base::CreateIntermediateMachPort(
         task_port, base::mac::ScopedMachSendRight(handle->port), nullptr);
     if (intermediate_port == MACH_PORT_NULL) {
@@ -111,6 +121,12 @@ bool MachPortRelay::ExtractPortRights(Channel::Message* message,
     if (handle->type != PlatformHandle::Type::MACH_NAME)
       continue;
 
+    if (handle->port == MACH_PORT_NULL) {
+      handle->type = PlatformHandle::Type::MACH;
+      num_received++;
+      continue;
+    }
+
     mach_port_t extracted_right = MACH_PORT_NULL;
     mach_msg_type_name_t extracted_right_type;
     kern_return_t kr =