I think "control" here is somewhat unreasonably pejorative.
They each do the vast majority of the dev work in their respective engines, so it's "control" only because if they want a feature they'll just implement it. Similarly neither implements things that aren't of interest to them - I can't speak for blink but webkit isn't going to oppose people contributing support for new features unless the implementation is not good (I assume this is a universal across all projects), or it's considered harmful (features that are inherently insecure - a la SVG raw socket access - or inherently harmful - e.g. new tech to aid tracking).
They each do the vast majority of the dev work in their respective engines, so it's "control" only because if they want a feature they'll just implement it. Similarly neither implements things that aren't of interest to them - I can't speak for blink but webkit isn't going to oppose people contributing support for new features unless the implementation is not good (I assume this is a universal across all projects), or it's considered harmful (features that are inherently insecure - a la SVG raw socket access - or inherently harmful - e.g. new tech to aid tracking).