Sanitize headers in Proxy Authentication Required responses

BUG=431504

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

Cr-Commit-Position: refs/heads/master@{#310014}
  const char* GetAlternateProtocolFromParam() {
    return
        AlternateProtocolToString(AlternateProtocolFromNextProto(GetParam()));
  }

  const char* GetAlternateProtocolFromParam() {
    return
        AlternateProtocolToString(AlternateProtocolFromNextProto(GetParam()));
  }
