.cdk-overlay-pane does not fit .cdk-overlay-container

#### Reproduction

Steps to reproduce:
1. Open https://stackblitz.com/edit/angular-overlay-scroll
2. Scroll body
3. Press 'overlay' button


#### Expected Behavior

`.cdk-overlay-pane` fits `.cdk-overlay-container` if `BlockScrollStrategy` is used.

#### Actual Behavior

`.cdk-overlay-pane` does not fit `.cdk-overlay-container` if `BlockScrollStrategy` is used.
A part of `.cdk-overlay-pane` is out `.cdk-overlay-container`.
There is no way to scroll to the bottom of the last `.overlay-items__item`.

#### Environment

- Angular: 7, 8
- CDK/Material: 7, 8
- Browser(s): Google Chrome, Firefox, Opera
- Operating System (e.g. Windows, macOS, Ubuntu): Windows, macOS, Android

#### Suggest a Fix
I have found two ways to fix it.
1. Make `OverlayConfig#minHeight` required
2. Set default value of `OverlayConfig#minHeight`

从链接的 StackBlitz 复现开始，跟踪 Angular CDK OverlayConfig 和 BlockScrollStrategy 在调整 pane 和 container 大小时涉及的行为。完成标准是：body 滚动后 overlay 能适应其 container，并且可以到达最后一个 overlay 项目的底部。

由索引模型根据 Issue 内容生成。