[Vertical Tabs] Add VerticalUnpinnedTabContainerView class.

Creates a new class to hold unpinned tabs and sets up layout. Updates
ScrollView to be able to use the its contents' preferred size (if not
empty) to set its bounds.

Bug: 438494724
Change-Id: I34b6e411266b196b1d1563ecfa37bb19c0ed5afc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6846862
Reviewed-by: David Pennington <dpenning@chromium.org>
Reviewed-by: Keren Zhu <kerenzhu@chromium.org>
Commit-Queue: Caroline Rising <corising@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1505134}