Remove most of MockTab casting

Follow up from discussion in https://crrev.com/c/4880465. It
was suggested that MockTab utilities should return a MockTab
so the caller doesn't have to cast to a MockTab. This
CL removes all but 1 instances of MockTab casting. The one
scenario where it's still necessary is in MockTabModelSelector
when overriding getCurrentTab().

Bug: b/302213679
Change-Id: I0131c14c7988158e1fd38729a7a4a854e9faa27b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4894914
Commit-Queue: David Maunder <davidjm@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1202801}