Fix confusing code that appeared to be causing UB

By the time ResetChromeIdentityServiceObserverForTesting() is
called, the old ChromeIdentityService has been destroyed thus
it is Undefined Behaviour to call RemoveObserver() via the
ScopedObserver::RemoveAll() method.

The code does not exhibit Undefined Behaviour because the
destructor of ChromeIdentityService notify all its observer
(via OnChromeIdentityServiceWillBeDestroyed) and they remove
themselves from the observer list.

Instead change ResetChromeIdentityServiceObserverForTesting()
to DCHECK() that the call to RemoveAll() has been performed
(via the OnChromeIdentityServiceWillBeDestroyed implementation)
by checking the ScopedObserver is not registered in any list.

Bug: 957973
Change-Id: Iccd019405f4972470fddbd9b7540232cc9bb3946
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1589955Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#655928}

ios/chrome/browser/signin/authentication_service.mm

@@ -424,7 +424,7 @@ bool AuthenticationService::ShowMDMErrorDialogForIdentity(
 }
 
 void AuthenticationService::ResetChromeIdentityServiceObserverForTesting() {
-  identity_service_observer_.RemoveAll();
+  DCHECK(!identity_service_observer_.IsObservingSources());
   identity_service_observer_.Add(
       ios::GetChromeBrowserProvider()->GetChromeIdentityService());
 }