[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.

StackBlitz の再現から始め、pageSize の直接代入と _changePageSize() を比較します。完了条件は、pageSize を変更すると、private メソッドを必要とせずに表示中の行が直ちに再描画され、1 ページあたりの項目数のテキストが更新されることです。

索引モデルが issue の本文から書いたものです。