Virtual Scrolling stalls when scrolling via keyboard with large set of complex data

#### Bug, feature request, or proposal: Bug

#### What is the expected behavior?
When scrolling down the list by holding down the up or down arrow key, the list should scroll smoothly all the way down to the end

#### What is the current behavior?
When scrolling down the list by holding down the up or down arrow key, at random times and seemingly random circumstances, the scrolling behavior stalls. I believe the focus is being lost on the list for whatever reason as, if you click on the list and press the arrow key, scrolling resumes as normal.

#### What are the steps to reproduce?
StackBlitz [here](https://stackblitz.com/edit/angular-material-yhc71v). Click on the list and press and hold the down arrow key. It may take a couple attempts, but eventually it should stall.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Reproduced in Chrome 71.0.3578.98 and IE 11. Could not reproduce in Firefox 64.0.

|Package|Version|
|---|---|
|@angular/animations|7.2.0|
|@angular/cdk|7.2.1|
|@angular/common|7.2.0|
|@angular/compiler|7.2.0|
|@angular/core|7.2.0|
|@angular/forms|7.2.0|
|@angular/http|7.2.0|
|@angular/material|7.2.1|
|@angular/platform-browser|7.2.0|
|@angular/platform-browser-dynamic|7.2.0|
|@angular/router|7.2.0|
|core-js|2.6.1|
|rxjs|6.3.3|
|rxjs-compat|6.3.3|
|zone.js|0.8.26|

#### Is there anything else we should know?
Resource-bound maybe?

Comienza con la reproducción vinculada de StackBlitz y confirma que mantener pulsada la flecha abajo acaba provocando que el desplazamiento se detenga en Chrome o IE, mientras que Firefox no se ve afectado. Rastrea la interacción del teclado y el comportamiento del foco de la lista con desplazamiento virtual a partir de esa reproducción. Se considera terminado cuando una lista grande de datos complejos se desplaza continuamente hasta el final con la tecla de flecha y la regresión está cubierta por una prueba adecuada.

Escrito por el modelo de indexación a partir del texto del issue.