Fix/tab close fixed width animation jump

[petemill]

Address feedback in #14477 (comment) which is really a regression report of #7001 via #11438, concerning jumpiness of tab fixed-width restore animation by:

• waiting a frame before animating tab width from forced width back to normal after closing tabs
• not forcing tab width when closing the last item (didn't make sense anyway since the mouse cursor cannot often still be at the close tab icon after closing the last tab in the strip, and the issue was caused by the mouse becoming in a window-draggable area which behaves erratically with mouse-leave)
• stopping to force tab width once a new tab is created (avoids tabs becoming too wide if a user closes some tabs with the mouse and then opens 1 or more new tabs, without moving the mouse away from the tabs toolbar.

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.
• Request a security/privacy review as needed. (Ask a Brave employee to help if you cannot access this document.)

Test Plan:

Reviewer Checklist:

• Request a security/privacy review as needed if one was not already requested.

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

[bsclifton]

Tested quite a few scenarios- this is working great 😄 Code changes look good too!

[bsclifton]

master d5ed260
0.24.x 6781909
0.23.x fc75570 @petemill can you help me merge this one to 0.23.x?

[petemill]

There was a conflict with 0.23.x which was in a weird state, almost as if it hadn't had a previous cherry-pick correctly.... fixed now though @bsclifton