The script previously used the PyPI package closure, which is slightly
out of date and not maintained by Google.
The JavaScript contribution docs and the compress.py script now runs the
google-closure-compiler package in the recommended way. Google's
documentation on usage and installation can be found at:
https://github.com/google/closure-compiler-npm/tree/master/packages/google-closure-compiler#usage
This also makes the usage simpler as the package now runs through npm's
npx utility, which will automatically install google-closure-compiler to
a per-user cache.
f48a1990fb