fix Brave crash on confirm bug

[willy-b]

for your consideration, just one way to fix this

• Submitted a ticket for my issue if one did not already exist. [HackerOne] window.confirm crashes Brave #4883
• Used Github auto-closing keywords in the commit message.
• Ran git rebase -i to squash commits (if needed).

Test Plan:

1. Open Brave
2. Visit http://pentesting.x10host.com (entire page content is: <script>confirm(1)</script>)
3. Observe that Brave does not crash, and confirm dialog with message content "1" is displayed.

Fix:

• dialog.showMessageBox requires that the message and title arguments are strings
• if they are numbers instead, they're now coerced to strings before passing them to showMessageBox

[willy-b]

(realized this can be solved by doing exactly what @bridiver did in #4798, so closing in favor of new PR, whoops.)