Fix Brave crashing on confirm bug

[willy-b]

(Realized that this can be solved with exactly what @bridiver did for alerts in #4798, just copied for confirm dialog, so replacing previous PR I had with @bridiver's sol'n)

• 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>, so can test with data:text/html,<script>confirm(1)</script> as well)
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
• make sure that when message is a number (or other non-string type) it is coerced to a string
• exact same change as what @bridiver did for alerts in Brave crashes if alert is called without a message #4798, but for confirm dialog

fixes #4883

[bbondy]

I think this can be removed now

if (title == null) {
    title = ''
}

[bbondy]

ditto here

[willy-b]

@bbondy, thanks for the review, I removed the extra lines. I can rebase and open a new PR or push directly to whatever branch you think is appropriate. Thanks!

[bbondy]

Perfect, I'll just do a squash merge, thanks.