[User Education] Simplify HelpBubbleHandlerBase interface

Start by updating the HelpBubbleHandlerBase::GetWebContents() comment.
Better explain when GetWebContents() would return nullptr, which is
only in tests that are not set up correctly. Offer developers reading
the code better advice on what to do in that case. Then follow the
updated comment and remove nullptr checks in all the GetWebContents()
callers.

As a result, VisibilityProvider::CheckIsVisible() can be simplified to
return a bool instead of a tri-state. Then delete test code that are no
longer applicable.

Change-Id: Idbd072bbe09a5190672d6c69ae5721804f663eef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7029560
Reviewed-by: Dana Fried <dfried@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1528287}