Revert of Allow ObserverListThreadSafe to be used from sequenced tasks. (patchset #23 id:440001 of https://codereview.chromium.org/2592143003/ )

Reason for revert:
This CL is causing tests to fail on Linux ChromiumOS Tests (dbg)(1):

- ProfileBrowserTest.OffTheRecordURLRequestContextIsolation
- ProfileBrowserTest.URLRequestContextIsolation

See example failed build:

https://luci-milo.appspot.com/buildbot/chromium.chromiumos/Linux%20ChromiumOS%20Tests%20%28dbg%29%281%29/22040

Original issue's description:
> Allow ObserverListThreadSafe to be used from sequenced tasks.
>
> Previously, observers could only be added to an ObserverListThreadSafe
> from single-threaded tasks. Observers were notified on the registration
> thread.
>
> With this CL, observers can also be added to an ObserverListThreadSafe
> from sequenced tasks. They are notified on the registration sequence.
>
> ObserverListThreadSafe behaves almost the same way as before when used
> from single-threaded tasks. The following things changed:
> - The order in which observers registered from the same thread
>   are notified is no longer deterministic.
> - One notification task is posted for each observer rather than one
>   notification task per thread.
> - If an observer is added to a NOTIFY_ALL ObserverListThreadSafe from
>   a stack of notification dispatches, only the notification on top of
>   the stack is sent to the newly added observer.
>
> BUG=675631
> TBR=rkaplow@chromium.org,melandory@chromium.org,bengr@chromium.org,sfiera@chromium.org
>
> Review-Url: https://codereview.chromium.org/2592143003
> Cr-Commit-Position: refs/heads/master@{#447311}
> Committed: https://chromium.googlesource.com/chromium/src/+/5062447d399be019735314f765d8d21b9433d995

TBR=gab@chromium.org,rkaplow@chromium.org,melandory@chromium.org,bengr@chromium.org,sfiera@chromium.org,fdoray@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=675631

Review-Url: https://codereview.chromium.org/2669673002
Cr-Commit-Position: refs/heads/master@{#447388}