Fix flaky LockManagedGuestSessionNotManagedGuestSession test

A TestExtensionRegistryObserver is used to wait for the in-session
extension to be ready before calling the Javascript test function.

Bug: 1075511
Change-Id: I77837f14782f63c69628e3364edaf0a41c99843a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2212277Reviewed-by: Alexander Hendrich <hendrich@chromium.org>
Commit-Queue: Jit Yao Yap <jityao@google.com>
Cr-Commit-Position: refs/heads/master@{#771746}

chrome/browser/chromeos/extensions/login_screen/login/login_apitest.cc

@@ -39,6 +39,7 @@
 #include "content/public/test/browser_test.h"
 #include "content/public/test/test_utils.h"
 #include "extensions/browser/api/test/test_api.h"
+#include "extensions/browser/test_extension_registry_observer.h"
 #include "extensions/common/extension.h"
 #include "extensions/common/manifest.h"
 #include "extensions/test/extension_test_message_listener.h"
@@ -146,6 +147,16 @@ class LoginApitest : public LoginScreenApitestBase {
     run_loop.Run();
   }
 
+  std::unique_ptr<extensions::TestExtensionRegistryObserver>
+  GetTestExtensionRegistryObserver(const std::string& extension_id) {
+    const user_manager::User* active_user =
+        user_manager::UserManager::Get()->GetActiveUser();
+    Profile* profile =
+        chromeos::ProfileHelper::Get()->GetProfileByUser(active_user);
+    return std::make_unique<extensions::TestExtensionRegistryObserver>(
+        extensions::ExtensionRegistry::Get(profile), extension_id);
+  }
+
   virtual void SetUpInSessionExtension() {
     std::unique_ptr<policy::UserPolicyBuilder> user_policy_builder =
         MakeInSessionExtensionUserPolicyBuilder();
@@ -157,12 +168,17 @@ class LoginApitest : public LoginScreenApitestBase {
     policy_data.set_settings_entity_id(kAccountId);
     user_policy_builder->Build();
 
+    auto registry_observer =
+        GetTestExtensionRegistryObserver(kInSessionExtensionId);
+
     ASSERT_TRUE(local_policy_mixin_.server()->UpdatePolicy(
         policy::dm_protocol::kChromePublicAccountPolicyType, kAccountId,
         user_policy_builder->payload().SerializeAsString()));
     session_manager_client()->set_device_local_account_policy(
         kAccountId, user_policy_builder->GetBlob());
     RefreshPolicies();
+
+    registry_observer->WaitForExtensionReady();
   }
 
   void SetTestCustomArg(const std::string custom_arg) {
@@ -402,6 +418,9 @@ class LoginApitestWithEnterpriseUser : public LoginApitest {
     policy_data.set_gaia_id(account_id.GetGaiaId());
     user_policy_builder->Build();
 
+    auto registry_observer =
+        GetTestExtensionRegistryObserver(kInSessionExtensionId);
+
     ASSERT_TRUE(
         logged_in_user_mixin_.GetLocalPolicyTestServerMixin()->UpdateUserPolicy(
             user_policy_builder->payload(), account_id.GetUserEmail()));
@@ -409,6 +428,8 @@ class LoginApitestWithEnterpriseUser : public LoginApitest {
         cryptohome::CreateAccountIdentifierFromAccountId(account_id),
         user_policy_builder->GetBlob());
     RefreshPolicies();
+
+    registry_observer->WaitForExtensionReady();
   }
 
  private:
@@ -441,9 +462,8 @@ IN_PROC_BROWSER_TEST_F(LoginApitestWithEnterpriseUser,
       kUnlockManagedGuestSessionNotManagedGuestSession);
 }
 
-// TODO(https://crbug.com/1075511) Flaky test
 IN_PROC_BROWSER_TEST_F(LoginApitestWithEnterpriseUser,
-                       DISABLED_LockManagedGuestSessionNotManagedGuestSession) {
+                       LockManagedGuestSessionNotManagedGuestSession) {
   LoginUser();
   SetUpTestListeners();
   SetUpInSessionExtension();