Sanitize headers in Proxy Authentication Required responses

BUG=431504

Review URL: https://codereview.chromium.org/769043003

Cr-Commit-Position: refs/heads/master@{#310014}
int HttpProxyClientSocket::DoDrainBody() {
  DCHECK(drain_buf_.get());
  DCHECK(transport_->is_initialized());
  next_state_ = STATE_DRAIN_BODY_COMPLETE;
  return http_stream_parser_->ReadResponseBody(
      drain_buf_.get(), kDrainBodyBufferSize, io_callback_);
}

int HttpProxyClientSocket::DoDrainBody() {
  DCHECK(drain_buf_.get());
  DCHECK(transport_->is_initialized());
  next_state_ = STATE_DRAIN_BODY_COMPLETE;
  return http_stream_parser_->ReadResponseBody(
      drain_buf_.get(), kDrainBodyBufferSize, io_callback_);
}
