DevTools: migrate treeoutline from hasChildren to is/setExpandable().

Source/devtools/front_end/sources/RevisionHistoryView.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 206 matching lines @@
     if (!allowRevert)
         this._revertElement.classList.add("hidden");
 }
 
 WebInspector.RevisionHistoryTreeElement.prototype = {
     onattach: function()
     {
         this.listItemElement.classList.add("revision-history-revision");
     },
 
-    onexpand: function()
+    onpopulate: function()
     {
         this.listItemElement.appendChild(this._revertElement);
 
-        if (this._wasExpandedOnce)
-            return;
-        this._wasExpandedOnce = true;
-
         this.childrenListElement.classList.add("source-code");
         if (this._baseRevision)
             this._baseRevision.requestContent(step1.bind(this));
         else
             this._revision.uiSourceCode.requestOriginalContent(step1.bind(this));
 
         /**
          * @param {?string} baseContent
          * @this {WebInspector.RevisionHistoryTreeElement}
          */
@@ skipping 84 matching lines @@
         contentSpan.classList.add("revision-history-line-" + changeType);
     },
 
     allowRevert: function()
     {
         this._revertElement.classList.remove("hidden");
     },
 
     __proto__: TreeElement.prototype
 }