Sticky columns are not correctly positionned when there are 2 sticky columns on the left

#### Reproduction

[Reproduced on stackblitz](https://stackblitz.com/edit/components-issue-8ut7gg)

Steps to reproduce:
1. Define a mat-table with 2 sticky columns on the left
2. Use the horizontal scrollbar
3. Position of the second column is not correctly computed

#### Expected Behavior

What behavior were you expecting to see?

Two sticky columns, correctly positioned and not moving when using the horizontal scrollbar.

#### Actual Behavior

What behavior did you actually see?

1. Sometimes, the second sticky column is actually moving when using the horizontal scrollbar (see the 3rd example on my stackblitz)
2. Sometimes the second sticky column is sticky, but not correctly positioned (see the 2nd example on my stackblitz: this column is too far on the right).

#### Environment

- Angular: 7, 8
- CDK/Material: 7, 8
- Browser(s): Firefox 71.0, Chromium 75.0.3770.142
- Operating System: Windows 10, Ubuntu 18.04

从链接的 StackBlitz 复现开始，关注 Angular Material mat-table 的 sticky-column 行为，重点检查水平滚动时左侧的两个 sticky columns。比较报告示例中第二列的位置，并在列出的 Firefox 和 Chromium 环境中验证结果。完成标准是两个 sticky columns 都保持固定，且第二列定位正确。

由索引模型根据 Issue 内容生成。