Remove callback DCHECK when retrieving dm token

Retrieve DM Token from BrowserDMTokenStorage doesn't have to wait for
async storing process finished because we are always reading from
memory.

Change-Id: I86aa534323fb73f736aacc05946448ac60c93293
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1931356Reviewed-by: Marc-André Decoste <mad@chromium.org>
Commit-Queue: Owen Min <zmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718291}

chrome/browser/policy/browser_dm_token_storage.cc

@@ -126,7 +126,6 @@ void BrowserDMTokenStorage::ClearDMToken(StoreCallback callback) {
 
 DMToken BrowserDMTokenStorage::RetrieveDMToken() {
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
-  DCHECK(!store_callback_);
 
   InitIfNeeded();
   return dm_token_;