This largely reduces the number of files needed to be compiled when we
change the MessagePort header.
(cherry picked from commit 32a22c49e3e999e681e2d1de1ebabd092a7f9b47)
Unfortunately the added test (which passes locally) is skipped as it is
based off other Worker tests which are also skipped due to being flakey
in CI.
(cherry picked from commit 755b63132b72cb25bd358415bf2fdf2be3c14c47)
Now that the global object correctly sets the property, we don't need
to do it here.
(cherry picked from commit 711faa8280ac4147f68ed778ad67047428329c67)
