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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Commencez par la reproduction StackBlitz liée et examinez comment les boîtes de dialogue ouvertes dans mat-sidenav-container appliquent les stratégies de défilement block et noop, avec et sans backdrop. Confirmez la correction en reproduisant les combinaisons signalées et en vérifiant que block empêche le défilement tandis que noop l’autorise.

Rédigé par le modèle d'indexation à partir du texte de l'issue.