Followups for "Explicitly prevent sandboxed navigation in the history interface"

#4787 by @dtapuska got merged with some issues that I did not spot. @bzbarsky came along after the merge and found the issues.

The conversation is currently happening over in #4787, but that's a closed pull request. Let's have this open issue tracking the fact that there's work to do. For now though, please see #4787 for discussion.