Fix log spam SigninScreenExtensionsExternalLoader

This fixes the following warning that was printed almost
unconditionally:

  "Ignoring untrusted value of the
  extensions.install.login_screen_extensions pref"

Also the CL removes unnecessary handling of the nullptr pref value,
as this case should never occur.

Bug: 1012892
Test: verify that no "Ignoring untrusted value" log appears in browser_tests --gtest_filter="SigninProfileExtensionsAutoUpdatePolicyTest.Test"
Change-Id: I41a18ae371b3b9a01fe9a6926731e5550fd0987f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2029507Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737464}

chrome/browser/chromeos/extensions/signin_screen_extensions_external_loader.cc

@@ -37,13 +37,8 @@ base::Value GetForceInstalledExtensionsFromPrefs(const PrefService* prefs) {
   const PrefService::Preference* const login_screen_extensions_pref =
       prefs->FindPreference(extensions::pref_names::kLoginScreenExtensions);
   CHECK(login_screen_extensions_pref);
-  const base::Value* login_screen_extensions_pref_value =
-      login_screen_extensions_pref->GetValue();
-  if (!login_screen_extensions_pref_value) {
-    // No sign-in screen extensions currently configured.
-    return base::Value(base::Value::Type::DICTIONARY);
-  }
-  if (!login_screen_extensions_pref->IsManaged()) {
+  if (!login_screen_extensions_pref->IsManaged() &&
+      !login_screen_extensions_pref->IsDefaultValue()) {
     // Ignore untrusted values - only the policy-specified setting is respected.
     // (This branch could be triggered if, for example, an attacker modified the
     // Local State file trying to inject some extensions into the Login Screen.)
@@ -51,6 +46,8 @@ base::Value GetForceInstalledExtensionsFromPrefs(const PrefService* prefs) {
                  << extensions::pref_names::kLoginScreenExtensions << " pref";
     return base::Value(base::Value::Type::DICTIONARY);
   }
+  const base::Value* login_screen_extensions_pref_value =
+      login_screen_extensions_pref->GetValue();
   DCHECK(login_screen_extensions_pref_value->is_dict());
   return login_screen_extensions_pref_value->Clone();
 }