Handle show-warning-dialog event on app

[darkdh]

requires brave/muon#196

fix #8846

Auditors: @bridiver, @bbondy

Submitter Checklist:

• 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).

Test Plan:

1. Open Firefox
2. Open Brave and import from Firefox
3. There will be a warning window

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run indivually or as a suite ref

[bsclifton]

++ 😄