Setting maxHeight on overlay with flexibleConnectedTo strategy does not work, when viewport margin is also configured

#### Reproduction

Steps to reproduce:
1. Visit the [stackblitz](https://stackblitz.com/edit/understanding-overlay-03-viewport-margin-bug)
2. Click the 2 buttons


#### Expected Behavior

That the max height of 300px is respected in both cases

#### Actual Behavior

Max height is only respected, if `withViewportMargin(10)` is not configured.

#### Environment

- Angular: 8.2.13
- CDK/Material: 8.2.3
- Browser(s): `Version 78.0.3904.70 (Official Build) (64-bit)`
- Operating System (e.g. Windows, macOS, Ubuntu): `macOS Mojave`

#### Gif


#### Relevant code

```typescript
const overlayRef = this.overlay.create({
hasBackdrop: true,
positionStrategy: this.overlay.position().flexibleConnectedTo(element)

// The max height is only respected, if the viewport margin is left out
.withViewportMargin(10) // <------

.withPositions([
{
originX: 'end',
originY: 'bottom',
overlayX: 'end',
overlayY: 'top',
},
]),
width: '200px',
maxHeight: '300px',
});
```

从链接的 StackBlitz 复现和使用 flexibleConnectedTo、withViewportMargin(10) 以及 maxHeight 的 overlay.create 示例开始。跟踪 flexible connected 定位如何处理 viewport 边距，并使用两个按钮验证修复，确保在每种情况下都遵守 300px 的最大高度限制。

由索引模型根据 Issue 内容生成。