Which browsers do support flow control feature of HTTP/2?
All browsers support flow control, it's a mandatory part of the spec. Section 5.2.1 of RFC 7540 explicitly says that "Flow control cannot be disabled.". I don't know of any browser that gives any sort of control to the javascript.
HTTP/2 clients for scripting languages such as Hyper (in python) or http-2 (in ruby) do give fine grained access to the protocol however.