Enforce/require MOJO_HANDLE_RIGHT_TRANSFER in sending handles via MojoWriteMessage().

mojo/edk/system/handle_table_unittest.cc

Index: mojo/edk/system/handle_table_unittest.cc
diff --git a/mojo/edk/system/handle_table_unittest.cc b/mojo/edk/system/handle_table_unittest.cc
index 92bffd7f97a739bec9ac68861b9c2a8d02664cee..19ef3ff4f7f284c3196ceae52be9a8080ed68a78 100644
--- a/mojo/edk/system/handle_table_unittest.cc
+++ b/mojo/edk/system/handle_table_unittest.cc
@@ -247,6 +247,8 @@ TEST(HandleTableTest, AddHandleVectorTooMany) {
     EXPECT_EQ(MOJO_RESULT_OK, handles[i].dispatcher->Close()) << i;
 }
 
+// TODO(vtl): Figure out how to test |MarkBusyAndStartTransport()|.
+
 }  // namespace
 }  // namespace system
 }  // namespace mojo