Add unittest for resume update

Test: treehugger
Bug: 173392916
Change-Id: I4b320685af93b20fc1f4abcdd6a416f1c97731ab
diff --git a/payload_consumer/delta_performer.h b/payload_consumer/delta_performer.h
index 7df2bd3..67ca6ce 100644
--- a/payload_consumer/delta_performer.h
+++ b/payload_consumer/delta_performer.h
@@ -170,6 +170,12 @@
   // Return true if header parsing is finished and no errors occurred.
   bool IsHeaderParsed() const;
 
+  // Checkpoints the update progress into persistent storage to allow this
+  // update attempt to be resumed after reboot.
+  // If |force| is false, checkpoint may be throttled.
+  // Exposed for testing purposes.
+  bool CheckpointUpdateProgress(bool force);
+
   // Compare |calculated_hash| with source hash in |operation|, return false and
   // dump hash and set |error| if don't match.
   // |source_fd| is the file descriptor of the source partition.
@@ -272,11 +278,6 @@
   // accordingly.
   void DiscardBuffer(bool do_advance_offset, size_t signed_hash_buffer_size);
 
-  // Checkpoints the update progress into persistent storage to allow this
-  // update attempt to be resumed after reboot.
-  // If |force| is false, checkpoint may be throttled.
-  bool CheckpointUpdateProgress(bool force);
-
   // Primes the required update state. Returns true if the update state was
   // successfully initialized to a saved resume state or if the update is a new
   // update. Returns false otherwise.