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

Reproduisez le comportement de l’exemple StackBlitz lié en ajoutant et en supprimant des éléments de la liste liée à cdk-virtual-scroll-viewport. Lisez la présentation générale du défilement de CDK et suivez le comportement de virtual-scroll concerné ; le travail est terminé lorsque le viewport reflète les modifications de la liste et que la documentation explique ce comportement.

Rédigé par le modèle d'indexation à partir du texte de l'issue.