Scrolling SDK (virtual-scroll) - View isn't updated when list is updated (items added/removed)

#### What is the expected behavior?
If a data-list that binded to ``, is updated (items are added/ removed from this list), the `` updated as well.

#### What is the current behavior?
Changing the list will not updated the view (``)

#### What are the steps to reproduce?
StackBlitz starter: https://stackblitz.com/edit/angular-hsr4tz?file=app%2Fcdk-virtual-scroll-overview-example.html

In this demo you can see that after adding items to the list, the view isn't updated.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular 7.1.4
Material 7.2.0
Browser: Chrome

#### Is there anything else we should know?
The documentation doesn't discuss about adding/removing items from data-list.
See docs at: https://material.angular.io/cdk/scrolling/overview

Riproduci il comportamento dell'esempio StackBlitz collegato aggiungendo e rimuovendo elementi dall'elenco associato a cdk-virtual-scroll-viewport. Leggi la panoramica dello scrolling di CDK e traccia il comportamento di virtual-scroll coinvolto; il lavoro è completato quando il viewport riflette le modifiche all'elenco e la documentazione spiega questo comportamento.

Scritto dal modello di indicizzazione a partire dal testo della issue.