Check Assistant allowed state for the opt-in flow

Bug: b/143516594
Test: Manual Test
Change-Id: I42251a3b7c97723c7eeb2f0267dbdc72a8fee66c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1888651Reviewed-by: Tao Wu <wutao@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Yue Li <updowndota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710914}

chrome/browser/ui/webui/chromeos/assistant_optin/assistant_optin_ui.cc

@@ -12,6 +12,7 @@
 #include "base/macros.h"
 #include "base/strings/string_number_conversions.h"
 #include "build/buildflag.h"
+#include "chrome/browser/chromeos/assistant/assistant_util.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/profiles/profile_manager.h"
 #include "chrome/browser/ui/ash/ash_util.h"
@@ -139,6 +140,14 @@ void AssistantOptInDialog::Show(
   return;
 #endif
 
+  // Check Assistant allowed state.
+  if (::assistant::IsAssistantAllowedForProfile(
+          ProfileManager::GetActiveUserProfile()) !=
+      ash::mojom::AssistantAllowedState::ALLOWED) {
+    std::move(callback).Run(false);
+    return;
+  }
+
   // Check session state here to prevent timing issue -- session state might
   // have changed during the mojom calls to launch the opt-in dalog.
   if (session_manager::SessionManager::Get()->session_state() !=