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[iOS] Have BVC get webStateList from Browser.

Greatly reduces BVC's dependence on TabModel by getting WebStateList
from Browser instead. This makes cleanup of various unit tests simpler,
as well as contributing to the removal of TabModel.

Remaining uses of TabModel in BVC all use TabUsageRecorder.

Bug: 1045575
Change-Id: I6850cd1b933b596b974e35cb535bee5db6f986ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2068605Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744265}