Fix rooting of composite reason when cancelling ReadableStream::tee

#34560 added a TODO and caused some tests to fail. We should use [[handle_value_array](https://github.com/jdm/servo/commit/53c2e16798299d58172fb2e6e8f9606dc3a1468d#diff-a1fc6164cda77dac6ce353deaf59404093bd84d024e0fec94a9919bef459d8c4R584)], when it is merged and use it to compose `reason_1` and `reason_2`.