sync: Attempt to recover from directory corruption

sync/syncable/on_disk_directory_backing_store.h

Index: sync/syncable/on_disk_directory_backing_store.h
diff --git a/sync/syncable/on_disk_directory_backing_store.h b/sync/syncable/on_disk_directory_backing_store.h
index f722ef95877b03cb5ae0242391d247c42307bd26..2b21681f91fe771f08de74bea294713f1355a449 100644
--- a/sync/syncable/on_disk_directory_backing_store.h
+++ b/sync/syncable/on_disk_directory_backing_store.h
@@ -21,7 +21,19 @@ class OnDiskDirectoryBackingStore : public DirectoryBackingStore {
       MetahandlesIndex* entry_bucket,
       Directory::KernelLoadInfo* kernel_load_info) OVERRIDE;
 
+  // A helper function that will make one attempt to load the directory.
+  // Unlike Load(), it does not attempt to recover from failure.
+  DirOpenResult TryLoad(
+      MetahandlesIndex* entry_bucket,
+      Directory::KernelLoadInfo* kernel_load_info);
+
+ protected:
+  void AllowFailureForTest();
+
  private:
+  FRIEND_TEST_ALL_PREFIXES(DirectoryBackingStoreTest, MinorCorruption);
+
+  bool allow_failure_for_test_;
   FilePath backing_filepath_;
 };