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This CL aims to remove all the references to SadTab (thus slim it down)
by factoring the related methods out to a separate class SadTab. It is
managed as Tab UserData, and takes care of creating/showing sad tab view
on ContentView. Also got SadTabFactory folded into it.

Note: Tab still has references to SadTab (isShowingSadTab) from
handleTabCrash/notifyContentChanged. Removing them involves somewhat
side-tracked changes as big as  the main one. Will handle it in
a follow-up CL.

Bug: 877878
Change-Id: I94a03e8f9999809cf690339a16570cc538e0e18c
Reviewed-on: https://chromium-review.googlesource.com/1235490
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594950}