Revert of Explicit notification and removal of lifecycle observers. (https://codereview.chromium.org/194923007/)

Reason for revert:
http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Blink&tests=fast/workers/storage/interrupt-database.html,http/tests/w3c/webperf/submission/Google/resource-timing/html/test_resource_dedicated_worker.html,http/tests/xmlhttprequest/workers/referer.html,http/tests/xmlhttprequest/workers/xmlhttprequest-response-type-blob.html

Original issue's description:
> Explicit notification and removal of lifecycle observers.
> 
> To help make disposing of execution contexts cleaner in a setting
> where both a context and its lifetime observers are being finalized
> by a garbage collector, add support for explicitly clearing out
> all lifecycle observers:
> 
> protected:
>   void LifecycleNotifier<T>::removeAndNotifyAllObservers()
> 
> Notifying them that the context has been destroyed while doing so.
> An operation that was previously (only) done by the LifecycleNotifier
> destructor.
> 
> Also provide a convenience method over ExecutionContext for clearing
> out its observers:
> 
>   void ExecutionContext::removeAllLifecycleObservers()
> 
> The operation is used here when shutting down and finalizing a worker
> thread and its global scope -- explicitly severing the connection
> of all observers to its execution context before performing the
> final garbage collection.
> 
> Doing so ensures that all observer objects being finalized during that
> garbage collection will not have access to its execution context object.
> A heap object that may or may not have been finalized by the garbage
> collector already.
> 
> R=haraken@chromium.org,ager@chromium.org,abarth@chromium.org
> BUG=340522
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=169435

TBR=oilpan-reviews@chromium.org,abarth@chromium.org,haraken@chromium.org,ager@chromium.org,eseidel@chromium.org,tkent@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=340522

Review URL: https://codereview.chromium.org/203393002

git-svn-id: svn://svn.chromium.org/blink/trunk@169444 bbb929c8-8fbe-4397-9dbb-9b2b20218538