Validate external protocols before launching on Windows

Bug: 889459
Change-Id: Id33ca6444bff1e6dd71b6000823cf6fec09746ef
Reviewed-on: https://chromium-review.googlesource.com/c/1256208
Reviewed-by: Greg Thompson <grt@chromium.org>
Commit-Queue: Mustafa Emre Acer <meacer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597611}
  ~OpenSystemSettingsHelper() {
    DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
  }

  ~OpenSystemSettingsHelper() {
    DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
  }
