sidenav: scroll strategies not working when mat-sidenav-container is used

#### What is the expected behavior?

When dialog is opened inside mat-sidenav-container scroll strategies should work as usual. Block should block scrolling when dialog is opened, noop should allow scrolling, etc.

#### What is the current behavior?

Scroll strategies don't work when mat-sidenav-container is used. If I open dialog with backdrop and noop scroll strategy, scrolling is still blocked. If I remove backdrop and use block strategy, scrolling is still possible.

#### What are the steps to reproduce?

https://stackblitz.com/edit/angular-material2-issue-effkee

Open dialog and try to scroll. Play around with backdrop and scroll settings.

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

Angular, Material and CDK 7.2.2, macos, Chrome (haven't tried other browsers but I assume it's the same)

#### Is there anything else we should know?

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リンクされた StackBlitz の再現例から始め、mat-sidenav-container 内で開かれたダイアログが、backdrop の有無に応じて block および noop のスクロール戦略をどのように適用するかを調査します。報告された組み合わせを再現し、block ではスクロールが防止され、noop ではスクロールが許可されることを確認して修正を検証します。

索引モデルが issue の本文から書いたものです。