[SyncFS] CountDirtyTracke() does not run a full scan

chrome/browser/sync_file_system/drive_backend/metadata_database_index_on_disk.h

Index: chrome/browser/sync_file_system/drive_backend/metadata_database_index_on_disk.h
diff --git a/chrome/browser/sync_file_system/drive_backend/metadata_database_index_on_disk.h b/chrome/browser/sync_file_system/drive_backend/metadata_database_index_on_disk.h
index 8dae9ce8fc1d930221d3d81f13c21aeb2c959528..440e9f4d871e6517d5757ea6567af14090a8a7cb 100644
--- a/chrome/browser/sync_file_system/drive_backend/metadata_database_index_on_disk.h
+++ b/chrome/browser/sync_file_system/drive_backend/metadata_database_index_on_disk.h
@@ -151,6 +151,9 @@ class MetadataDatabaseIndexOnDisk : public MetadataDatabaseIndexInterface {
   // Checks if |db_| has an entry whose key is |key|.
   bool DBHasKey(const std::string& key);
 
+  // Returns the number of dirty trackers, actually counting them.
+  size_t CountDirtyTrackerInternal() const;
+
   // Returns the number of entries starting with |prefix| in NumEntries format.
   // Entries for |ignored_id| are not counted in.
   NumEntries CountWithPrefix(const std::string& prefix, int64 ignored_id);
@@ -161,6 +164,8 @@ class MetadataDatabaseIndexOnDisk : public MetadataDatabaseIndexInterface {
   LevelDBWrapper* db_;  // Not owned.
   scoped_ptr<ServiceMetadata> service_metadata_;
 
+  size_t num_dirty_trackers_;
+
   DISALLOW_COPY_AND_ASSIGN(MetadataDatabaseIndexOnDisk);
 };