Convert mojo::system::Dispatcher to use our new refcounting stuff (instead of base's).

mojo/edk/system/message_in_transit.cc

Index: mojo/edk/system/message_in_transit.cc
diff --git a/mojo/edk/system/message_in_transit.cc b/mojo/edk/system/message_in_transit.cc
index 771c155cba4df3aaef5048e1438831b39668c99f..04d9e338393cfbde051eafbd419170dbfb064222 100644
--- a/mojo/edk/system/message_in_transit.cc
+++ b/mojo/edk/system/message_in_transit.cc
@@ -116,7 +116,7 @@ MessageInTransit::~MessageInTransit() {
       if (!(*dispatchers_)[i])
         continue;
 
-      DCHECK((*dispatchers_)[i]->HasOneRef());
+      (*dispatchers_)[i]->AssertHasOneRef();
       (*dispatchers_)[i]->Close();
     }
   }
@@ -151,8 +151,10 @@ void MessageInTransit::SetDispatchers(
 
   dispatchers_ = std::move(dispatchers);
 #ifndef NDEBUG
-  for (size_t i = 0; i < dispatchers_->size(); i++)
-    DCHECK(!(*dispatchers_)[i] || (*dispatchers_)[i]->HasOneRef());
+  for (size_t i = 0; i < dispatchers_->size(); i++) {
+    if ((*dispatchers_)[i])
+      (*dispatchers_)[i]->AssertHasOneRef();
+  }
 #endif
 }