Change emplace_back to push_back for external handlers

Inserting External Data Handlers into the vector of handlers
should use push_back rather than emplace_back since we're calling
std::make_unique instead of invoking the constructors directly.

Bug: chromium:1020443
Change-Id: I54b8f7394dfc571d49b486f4a39d7b506653488f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1893687Reviewed-by: Nikita Podguzov <nikitapodguzov@chromium.org>
Reviewed-by: Sergey Poromov <poromov@chromium.org>
Reviewed-by: Denis Kuznetsov [CET] <antrim@chromium.org>
Commit-Queue: Sean Kau <skau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712344}

chrome/browser/chromeos/login/users/chrome_user_manager_impl.cc

@@ -415,19 +415,19 @@ ChromeUserManagerImpl::ChromeUserManagerImpl()
     GetMinimumVersionPolicyHandler()->AddObserver(this);
   }
 
-  cloud_external_data_policy_handlers_.emplace_back(
+  cloud_external_data_policy_handlers_.push_back(
       std::make_unique<policy::UserAvatarImageExternalDataHandler>(
           cros_settings_, device_local_account_policy_service));
-  cloud_external_data_policy_handlers_.emplace_back(
+  cloud_external_data_policy_handlers_.push_back(
       std::make_unique<policy::WallpaperImageExternalDataHandler>(
           cros_settings_, device_local_account_policy_service));
-  cloud_external_data_policy_handlers_.emplace_back(
+  cloud_external_data_policy_handlers_.push_back(
       std::make_unique<policy::NativePrintersExternalDataHandler>(
           cros_settings_, device_local_account_policy_service));
-  cloud_external_data_policy_handlers_.emplace_back(
+  cloud_external_data_policy_handlers_.push_back(
       std::make_unique<policy::PrintServersExternalDataHandler>(
           cros_settings_, device_local_account_policy_service));
-  cloud_external_data_policy_handlers_.emplace_back(
+  cloud_external_data_policy_handlers_.push_back(
       std::make_unique<policy::CrostiniAnsiblePlaybookExternalDataHandler>(
           cros_settings_, device_local_account_policy_service));