bug(MatMenu): dropdown with menu items pops over menu trigger when there's a scroll bar in the dropdown

#### Reproduction

Steps to reproduce:
1. Go to the following StackBlitz URL https://stackblitz.com/edit/angular-xcgrfw
2. Click on the "Menu" button
3. Notice how menu items pop over the "Menu" button



#### Expected Behavior

The expected behaviour is for the menu dropdown to be positioned below the "Menu" button regardless of the number of menu items as in the following example https://stackblitz.com/angular/mnygrqvpejk?file=src%2Fapp%2Fmenu-overview-example.ts

#### Actual Behavior

The dropdown with menu items pops over the "Menu" button when there's so many menu items that the menu becomes scrollable

#### Environment

- Angular: 9.1.11
- CDK/Material: 9.2.4
- Browser(s): Google Chrome 83.0.4103.97
- Operating System (e.g. Windows, macOS, Ubuntu): Windows 10

首先使用链接的 StackBlitz 示例重现该问题，然后将其 MatMenu 行为与链接的 Angular 菜单示例进行比较。跟踪负责可滚动 dropdown 的菜单定位入口点，并验证当 dropdown 包含足够多、需要滚动的项目时，它仍保持在 trigger 下方。

由索引模型根据 Issue 内容生成。