> Requests with keepalive flag set are allowed to outlive the environment
> settings object. We want to make sure that such requests do not
> negatively impact the user experience when a page is unloaded, etc.
> This limits the amount of (body) bytes that can be inflight at any point
> when the request has the keepalive flag set; this flag is set by
Webkit should support the keepalive flag & enforce the inflight byte limit.