[DevTools] Remove remaining usages of global properties.

Source/devtools/front_end/ui/ViewportControl.js

 /*
  * Copyright (C) 2013 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 212 matching lines @@
      * @param {number} index
      * @return {number}
      */
     _cachedItemHeight: function(index)
     {
         return index === 0 ? this._cumulativeHeights[0] : this._cumulativeHeights[index] - this._cumulativeHeights[index - 1];
     },
 
     /**
      * @param {?Selection} selection
+     * @suppressGlobalPropertiesCheck
      */
     _isSelectionBackwards: function(selection)
     {
         if (!selection || !selection.rangeCount)
             return false;
         var range = document.createRange();
         range.setStart(selection.anchorNode, selection.anchorOffset);
         range.setEnd(selection.focusNode, selection.focusOffset);
         return range.collapsed;
     },
@@ skipping 358 matching lines @@
 
     /**
      * @return {number}
      */
     _visibleHeight: function()
     {
         // Use offsetHeight instead of clientHeight to avoid being affected by horizontal scroll.
         return this.element.offsetHeight;
     }
 }