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WebState's WasShown/WasHidden is called from Tab, so WebStateObserver
callbacks are actually work.
WebStateObserver's WasShown/WasHidden callbacks will allow to move
code from Tab to TabHelpers.

Bug: 616244
Change-Id: I2d8f3eb0f5854dd7204b9aec6db2aac593309638
Reviewed-on: https://chromium-review.googlesource.com/596783Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491739}