open background page devtools when navigating to background page url

[bridiver]

fix #7880
requires brave/muon@42b9c96

note: url will not properly reset until #7894 is fixed

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Ran git rebase -i to squash commits (if needed).

Test Plan:

1. Navigate to a background page url (chrome-extension://cjnmeadmgmiihncdidmfiabhenbggfjm/_generated_background_page.html)
2. page should not navigate
3. devtools should open for the background page
4. click on the main window to put the devtools in the background
5. navigate to the background page url again
6. devtools should focus
7. type console.log('test') in the devtools window
8. close the devtools
9. navigate to the background page again
10. devtools should open and display the previous output from console.log('test')

cc @jonathansampson @bbondy - please note the muon requirement for this

[bridiver]

cc @ayumi @diracdeltas - this fixes the issue with opening multiple copies of the background pages