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A TabLifecycleUnit represents a tab.

The implementation will be in a separate class (TabLifecyleUnitImpl)
because we want to expose only part of the it outside of
chrome/browser/resource_coordinator/.

E.g. the extensions system is allowed call:

TabLifecycleUnit::FromWebContents(contents)->Discard(condition);

but we don't want it to change the TabStripModel associated with
a tab (TabLifecycleUnitImpl::set_tab_strip_model()).

Bug: 775644
Change-Id: Idf2ad85bf2969871be1dc09eb1eb3776bc922959
Reviewed-on: https://chromium-review.googlesource.com/774979Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518410}