When loading https://httpbin.org/brotli with simplebrowser, it shows "Accept-Encoding": "gzip, deflate", indicating that there's no support for brotli encoding.

Chromium 57 sends "Accept-Encoding": "gzip, deflate, sdch, br", and according to caniuse this should be the case since 55.

FWIW, I found this in src/3rdparty/chromium/content/public/common/content_features.cc:

// Enables brotli "Accept-Encoding" advertising and "Content-Encoding" support.
// Brotli format specification: http://www.ietf.org/id/draft-alakuijala-brotli
const base::Feature kBrotliEncoding{"brotli-encoding",
                                    base::FEATURE_ENABLED_BY_DEFAULT};