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

components/sync/driver/sync_client.h

Index: components/sync/driver/sync_client.h
diff --git a/components/sync/driver/sync_client.h b/components/sync/driver/sync_client.h
index 77d8f33eefe7bfe7b5e00679fef9bcd8a82728a0..ddf64d83d97675a279f0ec22c5b5b687b7e78974 100644
--- a/components/sync/driver/sync_client.h
+++ b/components/sync/driver/sync_client.h
@@ -22,6 +22,10 @@ namespace autofill {
 class PersonalDataManager;
 }  // namespace autofill
 
+namespace base {
+class SequencedWorkerPool;
+}  // namespace base
+
 namespace bookmarks {
 class BookmarkModel;
 }  // namespace bookmarks
@@ -61,6 +65,10 @@ class SyncClient {
   // Initializes the sync client with the specified sync service.
   virtual void Initialize() = 0;
 
+  // Returns SequencedWorkerPool to be used by ProfileSyncService for blocking
+  // operations.
+  virtual base::SequencedWorkerPool* GetBlockingPool() = 0;
+
   // Returns the current SyncService instance.
   virtual SyncService* GetSyncService() = 0;