make number of tabs match max tabs per page

[cezaraugusto]

Auditors: @bbondy, @bsclifton
Fix #8909

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. Pin 3 tabs
2. Open tabs until a new tab set opens
3. Tab should focus on that new tab set
4. The number of unpinned tabs before new tab set opens should be the same as specified in settings

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

[bbondy]

💯 @cezaraugusto mentions he'll be writing a test for this here: #8972

[bsclifton]

++! 😄