[cdk-virtual-scroll-viewport][mat-table] - sticky headers is not working properly when combining mat-table with cdk-virtual-scroll-viewport

#### Reproduction

Use StackBlitz to reproduce your issue: https://stackblitz.com/fork/components-issue
https://stackblitz.com/edit/virtual-scroll-table-sticky-headers

Steps to reproduce:
1. wrap mat-table within a cdk-virtual-scroll-viewport element
2. add sticky: true property to *matHeaderRowDef directive as stated in documentation
3. observe sticky header slide down the viewport after about 10 rows.

#### Expected Behavior
Expect when adding the sticky:true; property to mat-table *matHeaderRowDef directive wrapped within a cdk-virtual-scroll-viewport that the headers remain positioned at the top of the viewport.

#### Actual Behavior
As you scroll the cdk-virtual-scroll-viewport slides the entire mat-table instead of just the row contents wrapped in the tbody element.

#### Environment

- Angular: >= 7
- CDK/Material: >= 7
- Browser(s): chrome, firefox, opera, safari
- Operating System (e.g. Windows, macOS, Ubuntu): all

Reproduce the problem using the linked StackBlitz examples with mat-table wrapped in cdk-virtual-scroll-viewport and sticky set on matHeaderRowDef. Investigate the interaction between the virtual scroll viewport and table header positioning; done means the header remains at the top while the table contents scroll across the listed browsers.

Scritto dal modello di indicizzazione a partire dal testo della issue.