Refactor supervised_user_interstitial_tab_closer to avoid tab_strip_model.h

Replace TabStripModel usage with
BrowserWindowInterface::GetAllTabInterfaces() to check tab count, and
remove the includes of browser.h and tab_strip_model.h. The tab count
check is equivalent, and the removed DCHECK on GetIndexOfWebContents was
redundant since FindBrowserWithTab already guarantees the web contents
belongs to the returned browser.

This prepares the file to be moved out of the monolith into
//chrome/browser/supervised_user:impl, where depending on
//chrome/browser/ui/tabs:tab_strip would create a circular dependency.

Bug: 353332589
Change-Id: Ia7ec35309ae067563ed6a044fc7d069d849de088
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7761321
Reviewed-by: Darryl James <dljames@chromium.org>
Commit-Queue: Tiago Vignatti <tvignatti@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1616469}