[SyncFS] Add completion callback to PromoteDemotedChanges (1/3)

chrome/browser/sync_file_system/local/local_file_sync_service.cc

Index: chrome/browser/sync_file_system/local/local_file_sync_service.cc
diff --git a/chrome/browser/sync_file_system/local/local_file_sync_service.cc b/chrome/browser/sync_file_system/local/local_file_sync_service.cc
index 6f0d0823ea73d8884739003d357973188b864ebd..f49d0007d537bfdc9de6c5f434dfb4716d157178 100644
--- a/chrome/browser/sync_file_system/local/local_file_sync_service.cc
+++ b/chrome/browser/sync_file_system/local/local_file_sync_service.cc
@@ -42,6 +42,12 @@ void PrepareForProcessRemoteChangeCallbackAdapter(
   callback.Run(status, sync_file_info.metadata, sync_file_info.changes);
 }
 
+void InvokeCallbackOnNthInvocation(int* count, const base::Closure& callback) {
+  --*count;
+  if (*count <= 0)
+    callback.Run();
+}
+
 }  // namespace
 
 LocalFileSyncService::OriginChangeMap::OriginChangeMap()
@@ -186,10 +192,20 @@ void LocalFileSyncService::HasPendingLocalChanges(
       origin_to_contexts_[url.origin()], url, callback);
 }
 
-void LocalFileSyncService::PromoteDemotedChanges() {
+void LocalFileSyncService::PromoteDemotedChanges(
+    const base::Closure& callback) {
+  if (origin_to_contexts_.empty()) {
+    callback.Run();
+    return;
+  }
+
+  base::Closure completion_callback =
+      base::Bind(&InvokeCallbackOnNthInvocation,
+                 base::Owned(new int(origin_to_contexts_.size())), callback);
   for (OriginToContext::iterator iter = origin_to_contexts_.begin();
        iter != origin_to_contexts_.end(); ++iter)
-    sync_context_->PromoteDemotedChanges(iter->first, iter->second);
+    sync_context_->PromoteDemotedChanges(iter->first, iter->second,
+                                         completion_callback);
 }
 
 void LocalFileSyncService::GetLocalFileMetadata(