I'm always confused about what gets shut down vs open sourced vs paid product.
We open sourced our open source policies/docs a while back, so if you're so inclined you can dig deeper there. These two links will be of particular interest: https://opensource.google.com/docs/creating/ and https://opensource.google.com/docs/why/
I can speak a little bit about what motivated us.
We saw from OSS-Fuzz (https://github.com/google/oss-fuzz) that this sort of thing could be widely useful and wanted non-open source code to benefit from making fuzzing easier.
For now, you can actually do the fuzzing on prem while communicating with app engine. We do this for our OS X bots since GCE doesn't offer OS X.
Aside: I tend to reach for node/js often for similar reasons (despite detractors) mostly because I'm more comfortable with it over Python or Ruby, but also because it's already integrated to most of the build/test environments I'm working on anyway.
Web apps, almost certainly no.
ClusterFuzz (and fuzzing generally) is most useful for finding bugs in C/C++ code so maybe it could work for unity games? I don't know much about them though.
But solidity was recently added to OSS-Fuzz: https://github.com/google/oss-fuzz/tree/master/projects/soli...
ClusterFuzz is infrastructure for running fuzzers, so we use it to run AFL, libFuzzer, and other domain specific fuzzers we've written.
Using it to run AFL gives us a lot of nice things over using AFL on someone's desktop (such as crash deduplication, automatic issue filing, fixed testing, regression ranges etc.)