Don't allow tab preview after close if config is off

[cezaraugusto]

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).
• Tagged reviewers and labelled the pull request as needed.

Issue first reported in #9306 (comment). cc @jonathansampson

Test Plan:

npm run test -- --grep="webview previews the next tab when current hovered tab is closed"

Manual Test Plan:

1. Disable tab previews
2. Open three tabs
3. Set first as active
4. Hover over second, close
5. Third should not be previewed
6. Enable tab preview again and start from 2.
7. It should preview

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[jonathansampson]

Concise. Appears to work. LGTM :)

[bsclifton]

Changes look good and test plan works 😄 👍

I did notice though that locally and on travis, one of the tests is failing:
https://travis-ci.org/brave/browser-laptop/jobs/244036926#L3204

Once that is resolved, this is ready for merge 😄

[cezaraugusto]

sorry changed frameKey just before pushing. updated.

[bsclifton]

Code works great and tests pass! Thanks, @cezaraugusto! 😄