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Reland of Clear LifecycleObserver::m_context when LifecycleObserver::contextDestroyed gets called (patchset #1 id:1 of https://codereview.chromium.org/2365463004/ )

Reason for revert:
Relanding this CL because the fix to the original crash has been landed in https://codereview.chromium.org/2364693005/

Original issue's description:
> Revert of Clear LifecycleObserver::m_context when LifecycleObserver::contextDestroyed gets called (patchset #11 id:200001 of https://codereview.chromium.org/2317483005/ )
>
> Reason for revert:
> Speculatively reverting for https://crbug.com/649272
>
> Original issue's description:
> > Clear LifecycleObserver::m_context when LifecycleObserver::contextDestroyed gets called
> >
> > This CL clears LifecycleObserver::m_context when LifecycleObserver::contextDestroyed gets called.
> > This means that LifecycleObserver::context() starts returning false after the context gets destroyed.
> >
> > The new behavior makes a lot of more sense, but the problem is that some existing code is assuming that
> > LifecycleObserver::context() keeps returning the context even after the context gets destroyed.
> > This CL fixed the problematic code to make all layout tests pass, but I'm not sure if I've updated all of the problematic code
> > (because the coverage of the layout tests is not sufficient).
> > However, the worst thing that can happen is that code that had been assuming that LifecycleObserver::context()
> > returns a context even after the context gets destroyed will stop working (it may cause a null-deref crash).
> > I think the risk is low.
> >
> > BUG=610176
> >
> > Committed: https://crrev.com/b729a998b05ec916731171d59de95e0aea31bbac
> > Cr-Commit-Position: refs/heads/master@{#419951}
>
> TBR=haraken@chromium.org
> # Not skipping CQ checks because original CL landed more than 1 days ago.
> BUG=610176,649272
> NOTRY=true
>
> Committed: https://crrev.com/04b1826356a37de2a322e7e6e0e80f07d05f210f
> Cr-Commit-Position: refs/heads/master@{#420498}

TBR=wfh@chromium.org,dcheng@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=610176,649272

Review-Url: https://codereview.chromium.org/2367753002
Cr-Commit-Position: refs/heads/master@{#420608}