code: function getWorkingSetSize(paneId) {
          var result = 0;
          _forEachPaneOrPanes(paneId, function (pane) {
              result += pane.getViewListSize();
          });
          return result;
      }
docs: Retrieves the size of the selected pane's view list
  @param {!string} paneId - id of the pane in which to get the workingset size.
  Can use `ALL_PANES` or `ACTIVE_PANE`
  @return {!number} the number of items in the specified pane