function getWorkingSetSize(paneId) {
        var result = 0;
        _forEachPaneOrPanes(paneId, function (pane) {
            result += pane.getViewListSize();
        });
        return result;
    }
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