Virtual Repeat Scroller doesn't shrink when items are removed

[colinskow]

I start out with the vertical repeat demo with 1000 items. Then I remove the last 900 items. Now, after I scroll past 100 it repeats the last page infinitely until it reaches the bottom of the original thousand.

Demonstration: https://codepen.io/colinskow/pen/PPmNbL

[colinskow]

Related to: #4950

[colinskow]

Got it fixed and am putting together a PR.

[colinskow]

VirtualRepeatContainerController.prototype.sizeScroller_ is getting called with NaN which causes the buggy behavior.

[colinskow]

Pull request: #5031