[IntentHandling] Remove the DCHECK_EQ for unique_ptr in App Service.

This CL removes the DECHECK_EQ for the unique_ptr because this is
a C++20 feature.

BUG=957519, 980914, 853604

Change-Id: I87c3213788ac4447aad2f3d46f8af2876ebf0053
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1910821Reviewed-by: Nancy Wang <nancylingwang@chromium.org>
Reviewed-by: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Commit-Queue: Maggie Cai <mxcai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714688}

chrome/services/app_service/app_service_impl.cc

@@ -237,11 +237,10 @@ void AppServiceImpl::ConnectToPrefService(
 
 void AppServiceImpl::OnPrefServiceConnected(
     std::unique_ptr<PrefService> pref_service) {
-  if (!pref_service) {
+  if (!pref_service || pref_service_) {
     // TODO(crbug.com/853604): Handle if not successfully connected.
     return;
   }
-  DCHECK_EQ(nullptr, pref_service_);
   pref_service_ = std::move(pref_service);
   InitializePreferredApps();
 }