Bug 1897322 - More consistently deal with pres shell style observers. r=smaug

Much like we deal with resize events.

Differential Revision: https://phabricator.services.mozilla.com/D210776

layout/base/PresShell.cpp

@@ -10217,6 +10217,7 @@ bool PresShell::RemovePostRefreshObserver(nsAPostRefreshObserver* aObserver) {
 }
 
 void PresShell::DoObserveStyleFlushes() {
+  MOZ_DIAGNOSTIC_ASSERT(!IsDestroying());
   MOZ_ASSERT(!ObservingStyleFlushes());
   mObservingStyleFlushes = true;