Migrate CBCMRegisterWatcher to new DM token class.

Migrate RetrieveDMToken call to use the new class.

Bug: 1020296
Change-Id: I8ed31825288333623e0b4669600b9b97da89436f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1903961Reviewed-by: Tien Mai <tienmai@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Commit-Queue: Dominique Fauteux-Chapleau <domfc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714208}

chrome/browser/policy/chrome_browser_cloud_management_register_watcher.cc

@@ -42,7 +42,7 @@ RegisterResult ChromeBrowserCloudManagementRegisterWatcher::
     return RegisterResult::kNoEnrollmentNeeded;
 
   // We are already enrolled successfully.
-  if (!token_storage->RetrieveDMToken().empty())
+  if (token_storage->RetrieveBrowserDMToken().is_valid())
     return RegisterResult::kEnrollmentSuccessBeforeDialogDisplayed;
 
   EnterpriseStartupDialog::DialogResultCallback callback = base::BindOnce(