[Sync] ModelTypeStore factory shouldn't require valid PSS to function correctly

components/sync/driver/fake_sync_client.cc

Index: components/sync/driver/fake_sync_client.cc
diff --git a/components/sync/driver/fake_sync_client.cc b/components/sync/driver/fake_sync_client.cc
index 698c01f65ad81a94b940153715aa64a80ad1fa70..8346be67677572e316b8da62aaebb8c8f1eca040 100644
--- a/components/sync/driver/fake_sync_client.cc
+++ b/components/sync/driver/fake_sync_client.cc
@@ -40,6 +40,10 @@ FakeSyncClient::~FakeSyncClient() {}
 
 void FakeSyncClient::Initialize() {}
 
+base::SequencedWorkerPool* FakeSyncClient::GetBlockingPool() {
+  return nullptr;
+}
+
 SyncService* FakeSyncClient::GetSyncService() {
   return sync_service_.get();
 }