Wrap native Assistant OptIn flow with buildflag

Bug: b/111262834
Test: Manual Test
Change-Id: Ifce7bd2e2a7be2d927301519b8e8375a326dd896
Reviewed-on: https://chromium-review.googlesource.com/1135912
Commit-Queue: Yue Li <updowndota@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574772}

chrome/browser/chromeos/BUILD.gn

@@ -74,6 +74,7 @@ source_set("chromeos") {
     "//chromeos:cryptohome_proto",
     "//chromeos:cryptohome_signkey_proto",
     "//chromeos:login_manager_proto",
+    "//chromeos/assistant:buildflags",
     "//chromeos/components/drivefs",
     "//chromeos/components/drivefs/mojom",
     "//chromeos/components/proximity_auth",

chrome/browser/chromeos/first_run/first_run.cc

@@ -25,6 +25,7 @@
 #include "chrome/common/chrome_switches.h"
 #include "chrome/common/extensions/extension_constants.h"
 #include "chrome/common/pref_names.h"
+#include "chromeos/assistant/buildflags.h"
 #include "chromeos/chromeos_switches.h"
 #include "components/arc/arc_prefs.h"
 #include "components/arc/arc_service_manager.h"
@@ -133,12 +134,14 @@ class DialogLauncher : public content::NotificationObserver {
         account_supported = true;
     }
 
+#if BUILDFLAG(ENABLE_CROS_LIBASSISTANT)
     // Launch Assistant OOBE flow if Assistant is enabled.
     if (account_supported && chromeos::switches::IsAssistantEnabled()) {
       chromeos::AssistantOptInDialog::Show();
       delete this;
       return;
     }
+#endif
 
     // If voice interaction value prop needs to be shown, the tutorial will be
     // shown after the voice interaction OOBE flow.