In https://github.com/servo/servo/pull/36977, when transferring
`TransformStream`, `CrossRealmTransform::Writable` and
`CrossRealmTransform::Readable` are set to different message ports. The
message port will not be readable and writable at the same time when
transferring the stream, so we can now merge
`cross_realm_transform_readable` and `cross_realm_transform_writable`
into a single field `cross_realm_transform`.
Testing: WPT ([passed on try
branch](4278417951))
Fixes: https://github.com/servo/servo/issues/37084
---------
Signed-off-by: Jason Tsai <git@pews.dev>