Create a strict order in ReadingListSpecifics

components/reading_list/ios/reading_list_entry.h

Index: components/reading_list/ios/reading_list_entry.h
diff --git a/components/reading_list/ios/reading_list_entry.h b/components/reading_list/ios/reading_list_entry.h
index 76648032c65f303c13ecf212443e2aa35ef15e1c..8a4fb3ddc50613169583fb329f1abe676f0d0ef6 100644
--- a/components/reading_list/ios/reading_list_entry.h
+++ b/components/reading_list/ios/reading_list_entry.h
@@ -90,20 +90,24 @@ class ReadingListEntry {
   static std::unique_ptr<ReadingListEntry> FromReadingListSpecifics(
       const sync_pb::ReadingListSpecifics& pb_entry);
 
-  // Merge the local data from |other| to this.
-  // The local fields (distilled_state_, distilled_url_, backoff_,
-  // failed_download_counter_) of |other| are moved to |this| and must not be
-  // used after this call.
-  void MergeLocalStateFrom(ReadingListEntry& other);
+  // Merge |this| and |other| into this.
+  // Local fields are kept from |this|.
+  // If |other.UpdateTime()| >= |this.UpdateTime()|, all fields are copied from
+  // |other|.
+  // If one of the state is UNSEEN, the other state is kept.
+  //
+  // After calling |MergeLocalStateFrom|, the result must verify
+  // ReadingListStore.CompareEntriesForSync(old_this.AsReadingListSpecifics(),
+  //                                        new_this.AsReadingListSpecifics())
+  // and
+  // ReadingListStore.CompareEntriesForSync(other.AsReadingListSpecifics(),
+  //                                        new_this.AsReadingListSpecifics()).
+  void MergeWithEntry(const ReadingListEntry& other);
 
   ReadingListEntry& operator=(ReadingListEntry&& other);
 
   bool operator==(const ReadingListEntry& other) const;
 
-  // Returns whether |lhs| is more recent than |rhs|.
-  static bool CompareEntryUpdateTime(const ReadingListEntry& lhs,
-                                     const ReadingListEntry& rhs);
-
   // Sets the title.
   void SetTitle(const std::string& title);
   // Sets the distilled URL and switch the state to PROCESSED and reset the time