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

Source/devtools/front_end/sources/RevisionHistoryView.js

Index: Source/devtools/front_end/sources/RevisionHistoryView.js
diff --git a/Source/devtools/front_end/sources/RevisionHistoryView.js b/Source/devtools/front_end/sources/RevisionHistoryView.js
index 0db7a3a36bb278fb14678590883fbc235ea6b2be..e32fc839196fda79cb16772092e5cc92b1fcf785 100644
--- a/Source/devtools/front_end/sources/RevisionHistoryView.js
+++ b/Source/devtools/front_end/sources/RevisionHistoryView.js
@@ -224,14 +224,10 @@ WebInspector.RevisionHistoryTreeElement.prototype = {
         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));