Reland "Move top-level files from chrome/browser/ash/login/ to namespace ash [2/2]"

This reverts commit da141a3858e3750a4e9d91d0e123157c8727ff98. This
reland fixes the build errors by adding a missing using declaration to
chromeos/system/statistics_provider.h.

Original change's description:
> Revert "Move top-level files from chrome/browser/ash/login/ to namespace ash [2/2]"
>
> This reverts commit 88a8086f87112b21011e75d09f75e97710fa145e.
>
> Reason for revert: Breaks the chromeos build and I (sheriff) had to close the tree until that revert land.
>
> See: https://logs.chromium.org/logs/chrome/buildbucket/cr-buildbucket/8835828362427336641/+/u/compile__with_patch_/raw_io.output_failure_summary_
> See:
> 'ash::system'; did you mean 'chromeos::system::kIsVmKey'
>
>
> Original change's description:
> > Move top-level files from chrome/browser/ash/login/ to namespace ash [2/2]
> >
> > The migration of chromeos/browser/chromeos/login/ was finished in
> > https://crrev.com/c/2722175. This CL changes the namespace from chromeos
> > to ash to the remaining files.
> >
> > This CL is part of the Chrome OS source code directory migration:
> > https://docs.google.com/document/d/1g-98HpzA8XcoGBWUv1gQNr4rbnD5yfvbtYZyPDDbkaE.
> >
> > Bug: 1164001
> > Change-Id: I774a7c33238512bf4f69735e95d5ee8de6ca9fa6
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3161996
> > Commit-Queue: Henrique Ferreiro <[email protected]>
> > Reviewed-by: Hidehiko Abe <[email protected]>
> > Owners-Override: Hidehiko Abe <[email protected]>
> > Cr-Commit-Position: refs/heads/main@{#922221}
>
> Bug: 1164001
> Change-Id: Id3577c9517c319606fbfa2032a83ffb12ba1da2a
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3168598
> Reviewed-by: Liviu Tinta <[email protected]>
> Owners-Override: Liviu Tinta <[email protected]>
> Cr-Commit-Position: refs/heads/main@{#922558}

Bug: 1164001
Change-Id: I299979feabdf5e84cbe0cc605fe5769a99cccc98
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3168781
Reviewed-by: Hidehiko Abe <[email protected]>
Owners-Override: Hidehiko Abe <[email protected]>
Commit-Queue: Henrique Ferreiro <[email protected]>
Cr-Commit-Position: refs/heads/main@{#923747}
diff --git a/chrome/browser/prefs/browser_prefs.cc b/chrome/browser/prefs/browser_prefs.cc
index 770b45b..4293ff1 100644
--- a/chrome/browser/prefs/browser_prefs.cc
+++ b/chrome/browser/prefs/browser_prefs.cc
@@ -997,8 +997,7 @@
   chromeos::language_prefs::RegisterPrefs(registry);
   chromeos::local_search_service::SearchMetricsReporter::
       RegisterLocalStatePrefs(registry);
-  chromeos::login::SecurityTokenSessionController::RegisterLocalStatePrefs(
-      registry);
+  ash::login::SecurityTokenSessionController::RegisterLocalStatePrefs(registry);
   ash::MultiProfileUserController::RegisterPrefs(registry);
   chromeos::NetworkMetadataStore::RegisterPrefs(registry);
   chromeos::NetworkThrottlingObserver::RegisterPrefs(registry);
@@ -1012,7 +1011,7 @@
   chromeos::SchedulerConfigurationManager::RegisterLocalStatePrefs(registry);
   ash::ServicesCustomizationDocument::RegisterPrefs(registry);
   chromeos::SigninScreenHandler::RegisterPrefs(registry);
-  chromeos::StartupUtils::RegisterPrefs(registry);
+  ash::StartupUtils::RegisterPrefs(registry);
   ash::StatsReportingController::RegisterLocalStatePrefs(registry);
   ash::system::AutomaticRebootManager::RegisterPrefs(registry);
   chromeos::TimeZoneResolver::RegisterPrefs(registry);
@@ -1312,8 +1311,7 @@
   ash::file_system_provider::RegisterProfilePrefs(registry);
   ash::full_restore::RegisterProfilePrefs(registry);
   ash::KerberosCredentialsManager::RegisterProfilePrefs(registry);
-  chromeos::login::SecurityTokenSessionController::RegisterProfilePrefs(
-      registry);
+  ash::login::SecurityTokenSessionController::RegisterProfilePrefs(registry);
   chromeos::multidevice_setup::MultiDeviceSetupService::RegisterProfilePrefs(
       registry);
   ash::MultiProfileUserController::RegisterProfilePrefs(registry);
@@ -1335,7 +1333,7 @@
   ash::SigninErrorNotifier::RegisterPrefs(registry);
   ash::ServicesCustomizationDocument::RegisterProfilePrefs(registry);
   chromeos::settings::OSSettingsUI::RegisterProfilePrefs(registry);
-  chromeos::StartupUtils::RegisterOobeProfilePrefs(registry);
+  ash::StartupUtils::RegisterOobeProfilePrefs(registry);
   ash::UserImageSyncObserver::RegisterProfilePrefs(registry);
   crostini::prefs::RegisterProfilePrefs(registry);
   ash::attestation::TpmChallengeKey::RegisterProfilePrefs(registry);