OOBE: Fix ash::GaiaScreen::LoadOnlineGaia crashes

Fix the crashes in GaiaScreen caused by accessing the pointer
returned from `BaseScreen::context()` while the screen is not shown.

The pointer returned from `BaseScreen::context()` is only set before
`GaiaScreen::ShowImpl()` is called and is reset after
`GaiaScreen::HideImpl()` is called. Outside this range the
`WizardContext` should be accessed from `LoginDisplayHost`.

Bug: b:306728962
Change-Id: Iecc4308d4b09f23cd835d0da4f57f6ae910cf114
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4961793
Reviewed-by: Denis Kuznetsov <antrim@chromium.org>
Commit-Queue: Osama Fathy <osamafathy@google.com>
Cr-Commit-Position: refs/heads/main@{#1213141}

chrome/browser/ash/login/screens/gaia_screen.cc

@@ -126,9 +126,10 @@ void GaiaScreen::LoadOnlineGaia() {
     return;
   }
 
-  switch (context()->gaia_config.gaia_path) {
+  auto* context = LoginDisplayHost::default_host()->GetWizardContext();
+  switch (context->gaia_config.gaia_path) {
     case WizardContext::GaiaPath::kDefault:
-      LoadDefaultOnlineGaia(context()->gaia_config.prefilled_account);
+      LoadDefaultOnlineGaia(context->gaia_config.prefilled_account);
       break;
     case WizardContext::GaiaPath::kReauth:
       LoadDefaultOnlineGaia(EmptyAccountId());
@@ -152,7 +153,9 @@ void GaiaScreen::LoadDefaultOnlineGaia(const AccountId& account) {
   if (base::CommandLine::ForCurrentProcess()->HasSwitch(
           switches::kForceCryptohomeRecoveryForTesting)) {
     DCHECK(features::IsCryptohomeRecoveryEnabled());
-    context()->gaia_config.gaia_path = WizardContext::GaiaPath::kReauth;
+    LoginDisplayHost::default_host()
+        ->GetWizardContext()
+        ->gaia_config.gaia_path = WizardContext::GaiaPath::kReauth;
     FetchGaiaReauthToken(account);
     return;
   }
@@ -172,8 +175,10 @@ void GaiaScreen::LoadDefaultOnlineGaia(const AccountId& account) {
 void GaiaScreen::Reset() {
   if (!view_)
     return;
-  context()->gaia_config.gaia_path = WizardContext::GaiaPath::kDefault;
-  context()->gaia_config.prefilled_account = EmptyAccountId();
+
+  auto* context = LoginDisplayHost::default_host()->GetWizardContext();
+  context->gaia_config.gaia_path = WizardContext::GaiaPath::kDefault;
+  context->gaia_config.prefilled_account = EmptyAccountId();
   view_->SetIsGaiaPasswordRequired(false);
   view_->Reset();
 }
@@ -330,7 +335,9 @@ void GaiaScreen::OnGetAuthFactorsConfiguration(
 
   const AccountId& account_id = user_context->GetAccountId();
   if (ShouldUseReauthEndpoint(account_id, is_recovery_configured)) {
-    context()->gaia_config.gaia_path = WizardContext::GaiaPath::kReauth;
+    LoginDisplayHost::default_host()
+        ->GetWizardContext()
+        ->gaia_config.gaia_path = WizardContext::GaiaPath::kReauth;
   }
 
   if (ShouldPrepareForRecovery(account_id) && is_recovery_configured) {