function _forEachPaneOrPanes(paneId, callback) {
        if (paneId === ALL_PANES) {
            _.forEach(_panes, callback);
        } else {
            callback(_getPane(paneId));
        }
    }
Iterates over the pane or ALL_PANES and calls the callback function for each.
@param {!string} paneId - id of the pane in which to adjust the scroll state, ALL_PANES or ACTIVE_PANE
@param {!function(!pane:Pane):boolean} callback - function to callback on to perform work.
The callback will receive a Pane and should return false to stop iterating.
@private