function cacheScrollState(paneId) {
        _forEachPaneOrPanes(paneId, function (pane) {
            _paneScrollStates[pane.id] = pane.getScrollState();
        });
    }
Caches the specified pane's current scroll state
If there was already cached state for the specified pane, it is discarded and overwritten
@param {!string} paneId - id of the pane in which to cache the scroll state,
                           ALL_PANES or ACTIVE_PANE