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?

从链接的 StackBlitz 复现开始，确认按住向下箭头最终会导致 Chrome 或 IE 中的滚动停滞，而 Firefox 不受影响。根据该复现追踪虚拟滚动列表的键盘交互和焦点行为。当大型复杂数据列表可以使用箭头键持续滚动到末尾，并且该回归由适当的测试覆盖时，即表示完成。

由索引模型根据 Issue 内容生成。