Typing in Autosize textarea on Firefox mobile causes unexpected characters to appear

#### Reproduction

Steps to reproduce:
1. Install Firefox mobile (https://play.google.com/store/apps/details?id=org.mozilla.firefox, versionName=68.4.2, versionCode=2015676155)
2. Navigate to https://material.angular.io/components/input/examples (currently version 8.2.3)
3. Scroll down to the the 'Auto-resizing textarea' section
4. Tap in the 'Autosize textarea' box
5. Type a few characters

#### Expected Behavior
Characters appear exactly as typed

#### Actual Behavior
- Extraneous characters appear.
- Looks like the Firefox mobile browser and Angular Material are fighting each other, causing extra characters to appear

#### Environment
- Angular: (whatever https://material.angular.io/components/input/examples is using)
- CDK/Material: 8.2.3
- Browser(s):
- Operating System (e.g. Windows, macOS, Ubuntu):

首先，在 Firefox mobile 68.4.2 中使用 Auto-resizing textarea，通过 Angular Material input examples page 重现该问题。然后，跟踪 Angular Material 和 CDK 源码中的 autosize textarea 行为。当输入的字符恰好只出现一次，且重现过程不再产生多余字符时，即表示完成。

由索引模型根据 Issue 内容生成。