Dynamic viewport height

#### Bug, feature request, or proposal:
Dynamically adjust the size of the table according to its content

#### What is the expected behavior?
It would be interesting to be able to define only the maxheight of the table and to adjust it dynamically according to the amount of data it contains until it reaches the maxheight. When the size is max height the virtualscrolling is available.

#### What is the current behavior?
When the cdk-virtual-scroll-viewport has no defined height it does not display all available values. If the height is defined the viewport displays all lines but when it is undefined just a few lines are displayed.

#### What are the steps to reproduce?
[Stackblitz](https://stackblitz.com/edit/angular-cdk-demo-virtual-scroll-jqpazr?embed=1&file=app/app.component.ts)

#### What is the use-case or motivation for changing an existing behavior?

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
latest cdk-experimental

#### Is there anything else we should know?

まず、リンクされた StackBlitz の再現を実行し、高さが未定義の場合の cdk-virtual-scroll-viewport の動作を調査します。テーブルが設定された最大の高さまでコンテンツに合わせて拡張し、その上限に達しても仮想スクロールを利用できれば、作業は完了です。

索引モデルが issue の本文から書いたものです。