Fix HeapSnapshotsDiff test, diff implementation, and a bug introduced

src/profile-generator.h

 // Copyright 2010 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 644 matching lines @@
   HeapSnapshot(HeapSnapshotsCollection* collection,
                Type type,
                const char* title,
                unsigned uid);
   ~HeapSnapshot();
 
   HeapSnapshotsCollection* collection() { return collection_; }
   Type type() { return type_; }
   const char* title() { return title_; }
   unsigned uid() { return uid_; }
-  HeapEntry* root() { return entries_[root_entry_index_]; }
+  HeapEntry* root() { return root_entry_; }
 
   void AllocateEntries(
       int entries_count, int children_count, int retainers_count);
   HeapEntry* AddEntry(
       HeapObject* object, int children_count, int retainers_count);
   bool WillAddEntry(HeapObject* object);
   HeapEntry* AddEntry(HeapEntry::Type type,
                       const char* name,
                       uint64_t id,
                       int size,
@@ skipping 21 matching lines @@
                       int children_count,
                       int retainers_count);
   HeapEntry* GetNextEntryToInit();
   static int GetObjectSize(HeapObject* obj);
   static int CalculateNetworkSize(JSObject* obj);
 
   HeapSnapshotsCollection* collection_;
   Type type_;
   const char* title_;
   unsigned uid_;
-  int root_entry_index_;
+  HeapEntry* root_entry_;
   char* raw_entries_;
   List<HeapEntry*> entries_;
   bool entries_sorted_;
   List<HeapEntryCalculatedData> calculated_data_;
 #ifdef DEBUG
   int raw_entries_size_;
 #endif
 
   friend class HeapSnapshotTester;
 
@@ skipping 306 matching lines @@
   friend class HeapSnapshotJSONSerializerIterator;
 
   DISALLOW_COPY_AND_ASSIGN(HeapSnapshotJSONSerializer);
 };
 
 } }  // namespace v8::internal
 
 #endif  // ENABLE_LOGGING_AND_PROFILING
 
 #endif  // V8_PROFILE_GENERATOR_H_