show about:blank in urlbar

[diracdeltas]

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

fix #5209

Auditors: @bbondy

Test Plan:

1. go to about:blank, observe that it is shown in the urlbar
2. highlight urlbar and go to about:newtab, observe that it is now empty

[bbondy]

great, thanks.