Sanitize headers in Proxy Authentication Required responses

BUG=431504

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

Cr-Commit-Position: refs/heads/master@{#310014}
  explicit BeforeNetworkStartHandler(bool defer)
      : defer_on_before_network_start_(defer),
        observed_before_network_start_(false) {}

  explicit BeforeNetworkStartHandler(bool defer)
      : defer_on_before_network_start_(defer),
        observed_before_network_start_(false) {}
