`alert()` dialogs on `chrome-extension://` origin (internal pages)

## Summary: Navigation to `chrome-extension` from the web is possible with #378805 (`ftp://` -> `chrome-extension://`). A blank page is created during navigation to `chrome-extension://` origin. Blank pages have "This page" title. It's possible to initiate `alert()` with a social-engineering content and "This page" title, that will be displayed on internal pages. ## Products affected: Brave: 0.23.31 V8: 6.7.288.46 rev: 3148acef36dba0fce89108638bb27927c4937f90 Muon: 7.1.5 OS Release: 17.6.0 Update Channel: Release OS Architecture: x64 OS Platform: macOS Node.js: 7.9.0 Brave Sync: v1.4.2 libchromiumcontent: 67.0.3396.103 ## Steps To Reproduce: 1. Start ftp server (sample ftp server attached, `npm i ftpd && node ftp-server.js`) 2. Open `ftp://localhost:7002/exploit.html` 3. Click "Go to payment settings" 4. Alert dialog with title "This page" will be displayed on `about:preferences#payments` page > And `ftp://localhost:7002/exploit.html` is blank, non-responsive and can't be reloaded. > adjust timer in `exploit.html` if it doesn't work ## Impact An attacker could initiate `alert()` with a social-engineering content and "This page" title, that will be displayed on internal pages.