SyncFS: Implement Get{Normal/Low}PriorityDirtyTracker in MetadataDatabase

chrome/browser/sync_file_system/drive_backend/metadata_database.h

Index: chrome/browser/sync_file_system/drive_backend/metadata_database.h
diff --git a/chrome/browser/sync_file_system/drive_backend/metadata_database.h b/chrome/browser/sync_file_system/drive_backend/metadata_database.h
index 02b44699649cf7fbcbb924a1393c5a6e32d47de8..7bd3c317b00fb685c275fcaf0b3550d69f58b1aa 100644
--- a/chrome/browser/sync_file_system/drive_backend/metadata_database.h
+++ b/chrome/browser/sync_file_system/drive_backend/metadata_database.h
@@ -247,11 +247,11 @@ class MetadataDatabase {
 
   // Returns true if there is a normal priority dirty tracker.
   // Assigns the dirty tracker if exists and |tracker| is non-NULL.
-  bool GetNormalPriorityDirtyTracker(FileTracker* tracker);
+  bool GetNormalPriorityDirtyTracker(FileTracker* tracker) const;
 
   // Returns true if there is a low priority dirty tracker.
   // Assigns the dirty tracker if exists and |tracker| is non-NULL.
-  bool GetLowPriorityDirtyTracker(FileTracker* tracker);
+  bool GetLowPriorityDirtyTracker(FileTracker* tracker) const;
 
  private:
   friend class ListChangesTaskTest;