It removes a script blocker.  Therefore, although it depends on the caller
whether it causes running script or not.  However, we should mark it as
`MOZ_CAN_RUN_SCRIPT` for safer code.

It's called only by the destructor of `nsAutoCauseReflowNotifier`.  Therefore,
this patch also marks its constructor as `MOZ_CAN_RUN_SCRIPT` for making
each creator method marked as `MOZ_CAN_RUN_SCRIPT` or
`MOZ_CAN_RUN_SCRIPT_BOUNDARY`.

Most of the creators is mutation listener methods.  However, `PresShell`
does nothing after destroying `nsAutoCauseReflowNotifier`.  Therefore,
this patch does not change the callers in MutationObserver.cpp to use
`RefPtr<PresShell>` at calling them because changing it may cause performance
regression.

Perhaps, we should create another methods of `WillCauseReflow()` and
`DidCauseReflow()` to avoid unnecessary `MOZ_CAN_RUN_SCRIPT` marking.
However, I'm not sure whether most callers may run script or not because
of outside of my knowledge.

Differential Revision: https://phabricator.services.mozilla.com/D55805

--HG--
extra : moz-landing-system : lando