virtual-scroll: memory leak in scroll listener

[mmalerba]

This is never unsubscribed

fromEvent(viewportEl, 'scroll')
            // Sample the scroll stream at every animation frame. This way if there are multiple
            // scroll events in the same frame we only need to recheck our layout once.
            .pipe(sampleTime(0, animationFrameScheduler))
            .subscribe(() => this._scrollStrategy.onContentScrolled());

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