[Paginator] Does not update correctly when `pageSize` updated programatically

#### Bug, feature request, or proposal:
Bug

#### What is the expected behavior?
When changing the `pageSize` attribute on a pager instance, the pager should immediately redraw the visible rows to match the new setting.

#### What is the current behavior?
The description text ("Items per page: 10") is updated, however the old number of rows still appear.

#### What are the steps to reproduce?
https://stackblitz.com/edit/angular-material2-issue-pvrjd5

#### What is the use-case or motivation for changing an existing behavior?
Dynamically changing the number of rows (e.g. on page resize)

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular 6.0.6, Material 6.4.1

#### Is there anything else we should know?
Calling `_changePageSize()` instead of changing the property works as expected.

从 StackBlitz 复现开始，并比较直接赋值 pageSize 与 _changePageSize()。完成的标准是：更改 pageSize 会立即重绘可见行，同时更新每页项目数文本，而无需调用私有方法。

由索引模型根据 Issue 内容生成。