tab-specific notifications should be closed when tab is closed

[gulkily]

Did you search for similar issues before submitting this one?
Yes

Describe the issue you encountered:
When I logged into chess.com, I ignored Brave's offer to remember my password. Now, every time I visit the site, the dialog is displayed again, even though I am not logging in again.

Expected behavior:
Dialog is displayed after logging in, but hidden after navigating away from the site.

• Platform (Win7, 8, 10? macOS? Linux distro?): macOS
• Brave Version: 0.11.4
• Steps to reproduce:
  1. Create a chess.com account (not via Facebook)
  2. Log into chess.com from the home page
  3. Verify that the dialog to remember the login creds is displayed at the top of the page. Do not click it.
  4. Close the chess.com tab
  5. Navigate to chess.com again

Expected: Prompt to remember password is not displayed again, unless I log in again

Actual: The prompt is displayed again as soon as I hit chess.com.

This is kind of alarming because it means Brave has remembered the password in memory even though I didn't ask it to save the password.

• Screenshot if needed: