[SxS] Ensure tab modals are shown and hidden

- TDM should observe visible/hidden because the dialog should be
hidden even when a non-active tab becomes hidden. This is a no-op for
non-split view but fixes split view
- TDM can show dialogs on visible but inactive tabs in splits in the
event that both tabs in a split have a dialog
- TabModel needs to notify the DidBecomeVisible callback when the
inactive tab in a split becomes visible
- TabStripModel needs to notify when all foreground tabs enter will
enter the background

Bug: 445758928
Change-Id: Iabd4505b7f02729e1e296612d466f779c7f16a91
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6962592
Reviewed-by: Eshwar Stalin <estalin@chromium.org>
Commit-Queue: Alison Gale <agale@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1518984}