Fixes flakiness caused by https://crrev.com/c/2909862

This CL fixes flakiness in
ProfileBrowserTestWithoutDestroyProfile.DestroyRegularProfileBeforeOTRs.
Before changing the pref value we should also make sure prefs_ exist
and ShutdownOnUIThread wasn't called.

Bug: 1217988
Change-Id: I5f9ccc7c28710f174d03aeef78aad33954dbb950
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2953346
Commit-Queue: Viktor Semeniuk <vsemeniuk@google.com>
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#892131}

components/password_manager/core/browser/password_store.cc

@@ -1328,7 +1328,7 @@ bool PasswordStore::RemoveOldGoogleLogins() {
 }
 
 void PasswordStore::MarkOldGoogleLoginsRemoved(bool success) {
-  if (success) {
+  if (success && prefs_ && !shutdown_called_) {
     prefs_->SetBoolean(prefs::kWereOldGoogleLoginsRemoved, true);
   }
 }