Add more checks to investigate SupervisedUserPrefStore crash at startup.

chrome/browser/prefs/chrome_pref_service_factory.cc

Index: chrome/browser/prefs/chrome_pref_service_factory.cc
diff --git a/chrome/browser/prefs/chrome_pref_service_factory.cc b/chrome/browser/prefs/chrome_pref_service_factory.cc
index 6b7fd8acf578f3fae6f58878a81905c64786ebe1..3581fe1134e670e4b05a51b8300d77385000a18f 100644
--- a/chrome/browser/prefs/chrome_pref_service_factory.cc
+++ b/chrome/browser/prefs/chrome_pref_service_factory.cc
@@ -441,9 +441,12 @@ void PrepareFactory(
 
 #if defined(ENABLE_SUPERVISED_USERS)
   if (supervised_user_settings) {
-    factory->set_supervised_user_prefs(
-        make_scoped_refptr(
-            new SupervisedUserPrefStore(supervised_user_settings)));
+    scoped_refptr<PrefStore> supervised_user_prefs = make_scoped_refptr(
+        new SupervisedUserPrefStore(supervised_user_settings));
+    // TODO(bauerb): Temporary CHECK while investigating
+    // https://crbug.com/425785. Remove when that bug is fixed.
+    CHECK(async || supervised_user_prefs->IsInitializationComplete());
+    factory->set_supervised_user_prefs(supervised_user_prefs);
   }
 #endif