DevTools: hide padding rows by default in datagrid viewport.

Source/devtools/front_end/HeapSnapshotDataGrids.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 562 matching lines @@
     __proto__: WebInspector.HeapSnapshotSortableDataGrid.prototype
 }
 
 /**
  * @constructor
  */
 WebInspector.HeapSnapshotPaddingNode = function()
 {
     this.element = document.createElement("tr");
     this.element.classList.add("revealed");
+    this.setHeight(0);
 }
 
 WebInspector.HeapSnapshotPaddingNode.prototype = {
    setHeight: function(height)
    {
        this.element.style.height = height + "px";
    },
    removeFromTable: function()
    {
         var parent = this.element.parentNode;
@@ skipping 529 matching lines @@
                 return compareResult;
             if (a[fieldName] < b[fieldName])
                 return -compareResult;
             return 0;
         }
         return compare;
      },
 
     __proto__: WebInspector.HeapSnapshotViewportDataGrid.prototype
 }