fix: unregisterScrollListener error in multiple instances of Windowscroller

[lsky-walt]

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When multiple WindowScrollers are bound to the same Dom, unregisterScrollListener error occurs.

*Example: *

<WindowScroller
  ref={this._setRef}
  scrollElement={isScrollingCustomElement ? customElement : window}>
  {({height, isScrolling, registerChild, onChildScroll, scrollTop}) => (
    <div className={styles.WindowScrollerWrapper}>
      <AutoSizer disableHeight>
        {({width}) => (
          <div ref={registerChild}>
            <List
              ref={el => {
                window.listEl = el;
              }}
              autoHeight
              className={styles.List}
              height={height}
              isScrolling={isScrolling}
              onScroll={onChildScroll}
              overscanRowCount={2}
              rowCount={list.size}
              rowHeight={30}
              rowRenderer={this._rowRenderer}
              scrollToIndex={scrollToIndex}
              scrollTop={scrollTop}
              width={width}
            />
          </div>
        )}
      </AutoSizer>
    </div>
  )}
</WindowScroller>

<h1 style={{textAlign: 'center', height: 300, backgroundColor: 'silver', color: 'white', lineHeight: '300px'}}>Second WindowScroller</h1>

<WindowScroller
  ref={this._setRef}
  scrollElement={isScrollingCustomElement ? customElement : window}>
  {({height, isScrolling, registerChild, onChildScroll, scrollTop}) => (
    <div className={styles.WindowScrollerWrapper}>
      <AutoSizer disableHeight>
        {({width}) => (
          <div ref={registerChild}>
            <List
              ref={el => {
                window.listEl = el;
              }}
              autoHeight
              className={styles.List}
              height={height}
              isScrolling={isScrolling}
              onScroll={onChildScroll}
              overscanRowCount={2}
              rowCount={list.size}
              rowHeight={30}
              rowRenderer={this._rowRenderer}
              scrollTop={scrollTop}
              width={width}
            />
          </div>
        )}
      </AutoSizer>
    </div>
  )}
</WindowScroller>

[Aegz]

Hey @lsky-walt , do you want to jump in on my PR that does the same thing but has unit tests?

https://github.com/bvaughn/react-virtualized/pull/1660