This directory contains the interface and implementation of the API to use an external network stack from gRPC. The implementation is essentially a thin wrapper around net::BidirectionalStream. The API specifies that the caller to gRPC will pass in an opaque binary blob (stream_engine) that can be used to created binary streams. In Chromium, this binary blob is a net::URLRequestContextGetter, which is used by grpc_support::BidirectionalStream to drive a net::BidirectionalStream.
Currently Cronet (//components/cronet/ios) is the only consumer of this API, but eventually code inside of Chromium should be able to use gRPC by providing a net::URLRequestContextGetter.