Refactor Chrome Labs to use singleton pattern and remove model parameters

- Update ShouldShowChromeLabsUI() to not require model parameter,
enabling proper browser action visibility setting during creation.

- Convert ChromeLabsModel to singleton pattern using base::NoDestructor.

- Remove all individual ChromeLabsModel instances from ToolbarView,
AppMenuModel, ChromeLabsCoordinator, and ChromeLabsViewController.

- Update UpdateChromeLabsNewBadgePrefs() and AreNewChromeLabsExperi-
mentsAvailable() to use singleton pattern.

Bug: 354758327
Change-Id: Idca17ce4649bc6b36dff53deeffd318bf89e2459
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6780340
Reviewed-by: Tom Lukaszewicz <tluk@chromium.org>
Commit-Queue: Ho Cheung <hocheung@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1490650}