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

mojo/edk/system/handle_table.cc

Index: mojo/edk/system/handle_table.cc
diff --git a/mojo/edk/system/handle_table.cc b/mojo/edk/system/handle_table.cc
index e123ae9bb83a7adf3154799f5bf2d284f22097cd..985e4edf53b5b070d362419a7d5333fc3b63667b 100644
--- a/mojo/edk/system/handle_table.cc
+++ b/mojo/edk/system/handle_table.cc
@@ -143,6 +143,11 @@ MojoResult HandleTable::MarkBusyAndStartTransport(
       error_result = MOJO_RESULT_BUSY;
       break;
     }
+    // Note: "Busy" is "preferred" over "permission denied".
+    if (!entries[i]->handle.has_all_rights(MOJO_HANDLE_RIGHT_TRANSFER)) {
+      error_result = MOJO_RESULT_PERMISSION_DENIED;
+      break;
+    }
     // Note: By marking the handle as busy here, we're also preventing the
     // same handle from being sent multiple times in the same message.
     entries[i]->busy = true;