external protocol prompt does not appear on about: pages

[diracdeltas]

STR:

1. go to about:newtab (or just open a new tab)
2. enter 'mailto:test' in the url bar
3. note that the mail app opens without showing a permission prompt

whereas if you do step 2 on a page such as google.com, the permission prompt appears as expected.

EDIT: marked as tor due to https://hackerone.com/bugs?subject=brave&report_id=388494

[diracdeltas]

i thought we fixed this at some point so maybe this is a regression. can't find the original issue though.

[btlechowski]

Original issue: #13643.

[diracdeltas]

https://hackerone.com/bugs?subject=brave&report_id=388494 makes this more of a serious issue