[SyncFS] Check MetadataDatabase availability before database operation

chrome/browser/sync_file_system/drive_backend/sync_engine.cc

Index: chrome/browser/sync_file_system/drive_backend/sync_engine.cc
diff --git a/chrome/browser/sync_file_system/drive_backend/sync_engine.cc b/chrome/browser/sync_file_system/drive_backend/sync_engine.cc
index 6f687d76f5b779538a527f9f2898e853b478ab20..f6f7e98bacda7a8662062267a21b1106c0d6b812 100644
--- a/chrome/browser/sync_file_system/drive_backend/sync_engine.cc
+++ b/chrome/browser/sync_file_system/drive_backend/sync_engine.cc
@@ -290,7 +290,8 @@ void SyncEngine::DownloadRemoteVersion(
 }
 
 void SyncEngine::PromoteDemotedChanges() {
-  metadata_database_->PromoteLowerPriorityTrackersToNormal();
+  if (metadata_database_)
+    metadata_database_->PromoteLowerPriorityTrackersToNormal();
 }
 
 void SyncEngine::ApplyLocalChange(
@@ -428,12 +429,18 @@ SyncEngine::SyncEngine(
 
 void SyncEngine::DoDisableApp(const std::string& app_id,
                               const SyncStatusCallback& callback) {
-  metadata_database_->DisableApp(app_id, callback);
+  if (metadata_database_)
+    metadata_database_->DisableApp(app_id, callback);
+  else
+    callback.Run(SYNC_STATUS_OK);
 }
 
 void SyncEngine::DoEnableApp(const std::string& app_id,
                              const SyncStatusCallback& callback) {
-  metadata_database_->EnableApp(app_id, callback);
+  if (metadata_database_)
+    metadata_database_->EnableApp(app_id, callback);
+  else
+    callback.Run(SYNC_STATUS_OK);
 }
 
 void SyncEngine::PostInitializeTask() {