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

Reproduziere das Verhalten im verlinkten StackBlitz-Beispiel, indem du Elemente zur an cdk-virtual-scroll-viewport gebundenen Liste hinzufügst und daraus entfernst. Lies die Übersicht zum CDK-Scrolling und verfolge das dabei relevante Virtual-Scroll-Verhalten; abgeschlossen ist die Aufgabe, wenn das Viewport Änderungen an der Liste widerspiegelt und die Dokumentation dieses Verhalten erklärt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.