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As the -dealloc is called at a later time with ARC, move the cleanup
from -dealloc to -browserStateDestroyed to ensure it is determinically
called even if ARC decide to -autorelease TabModel.

Cleanup some of Tab ivar when the WebState is destroyed so that the
corresponding Tab helpers -dealloc is called sooner.

BUG=None

Review-Url: https://codereview.chromium.org/2885803002
Cr-Commit-Position: refs/heads/master@{#473892}