[SxS] Make the SplitTabsButton pinnable

Add a new pref to track if the SplitTabsButton should be pinned to the
toolbar and show/hide the button accordingly.

Fixed: 403350621
Change-Id: I450a3b8d7cceebb0f6e5f8cd5e3236e67fb4e689
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6434383
Commit-Queue: Steven Luong <stluong@chromium.org>
Reviewed-by: Rushan Suleymanov <rushans@google.com>
Reviewed-by: Eshwar Stalin <estalin@chromium.org>
Reviewed-by: Elly FJ <ellyjones@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1445950}