AsyncCallTracker::contextDestroyed should not call clearLifecycleContext()
AsyncCallTracker::contextDestroyed should not call clearLifecycleContext()

This is a speculative fix for bug 451072 and probably for 450493.

If we clear LifecycleObserver::m_context in AsyncCallTracker::contextDestroyed, the following scenario can happen:

(1) AsyncCallTracker::contextDestroyed clears LifecycleObserver::m_context.
(2) ~AsyncCallTracker calls ~LifecycleObserver, but ~LifecycleObserver does not unregister the LifecycleObserver from the LifecycleContext.
(3) ~LifecycleContext() iterates the LifecycleObserver and crashes.

This CL fixes the crash by not calling clearLifecycleContext in AsyncCallTracker::contextDestroyed and thus have ~AsyncCallTracker unregister the observer from the LifecycleContext.

BUG=451072

Review URL: https://codereview.chromium.org/864143003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@188945 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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