Make "three snapshot technique" work on heap snapshots loaded from files

Source/devtools/front_end/JSHeapSnapshot.js

 /*
  * Copyright (C) 2012 Google Inc. 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
@@ skipping 30 matching lines @@
         pageObject: 4, // The idea is to track separately the objects owned by the page and the objects owned by debugger.
 
         visitedMarkerMask: 0x0ffff, // bits: 0,1111,1111,1111,1111
         visitedMarker:     0x10000  // bits: 1,0000,0000,0000,0000
     };
     this._lazyStringCache = { };
     WebInspector.HeapSnapshot.call(this, profile, progress);
 }
 
 WebInspector.JSHeapSnapshot.prototype = {
+    maxJsNodeId: function()
+    {
+        var nodeFieldCount = this._nodeFieldCount;
+        var nodes = this._nodes;
+        var nodesLength = nodes.length;
+        var id = 0;
+        for (var nodeIndex = this._nodeIdOffset; nodeIndex < nodesLength; nodeIndex += nodeFieldCount) {
+            var nextId = nodes[nodeIndex];
+            // JS objects have odd ids, skip native objects.
+            if (nextId % 2 === 0)
+                continue;
+            if (id < nodes[nodeIndex])
+                id = nodes[nodeIndex];
+        }
+        return id;
+    },
+
     createNode: function(nodeIndex)
     {
         return new WebInspector.JSHeapSnapshotNode(this, nodeIndex);
     },
 
     createEdge: function(edges, edgeIndex)
     {
         return new WebInspector.JSHeapSnapshotEdge(this, edges, edgeIndex);
     },
 
@@ skipping 614 matching lines @@
     },
 
     isWeak: function()
     {
         return this._edge().isWeak();
     },
 
     __proto__: WebInspector.HeapSnapshotRetainerEdge.prototype
 }