code: function restoreMetaViewport ( _window ) {
      var el;
  
      _window || ( _window = window );
  
      if ( _originalMetaViewport[_window] !== undefined ) {
  
          if ( _originalMetaViewport[_window] ) {
              // A setting has been captured
              el = ensureMetaViewport( _window );
              el.content = _originalMetaViewport[_window];
          } else {
              // The tag did not exist previously but has been created and manipulated. (The _originalMetaViewport entry
              // for the _window is set to "", not undefined, in this case.)
              ensureDefaultMetaViewport();
          }
  
          // Remove the captured setting.
          delete _originalMetaViewport[_window];
      }
  }
docs: Restores the original meta viewport tag after it has been modified by setMetaViewportZoom(). Does nothing if there is
  no preceding setMetaViewportZoom() call to be reverted.
  
  If the meta viewport tag didn't exist initially, but has been created by setMetaViewportZoom(), it does not get
  deleted. It is reset to the default meta tag instead.
  
  NB If the tag did not exist previously, deleting it would be true to the original HTML - but it would also be
  misleading. Browsers do not revert to their original state. According to Peter Paul Koch, the tag can't be unset by
  removing it. See Peter Paul Koch, The Mobile Web Handbook, Ch. 3 "Viewports", Section "Changing the Meta Viewport
  Tag".
  
  @param   {Window} [_window=window]