Add a test to ensure that net stack accepts brotli encoded content from DRP proxy.

Network stack wants to add checks which will reject responses with encoding type that do not match the advertised encoding type (https://groups.google.com/a/chromium.org/d/topic/chromium-dev/GxIgxok9f2k/discussion). The plan is to add these checks below the proxy delegate. So, DRP usage of Brotli should remain unaffected.

To ensure that DRP usage remains unaffected, DRP should add additional tests that ensure that Brotli encoded content from DRP is successfully handled.