cdk-global-scrollblock not removed when navigating while dialog is being closed

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

#### What is the expected behavior?
cdk-global-scrollblock is removed from document when the router navigates to a new route while the dialog is being closed (not yet tested if the same happens while navigating when a dialog is open)

#### What is the current behavior?
When navigating while closing the dialog, the class cdk-global-scrollblock will not be removed from document and scrolling is not possible anymore.

#### What are the steps to reproduce?
Open a dialog, call this.mdDialogRef.close() and immediately navigate to a different route. It can be solved easily by observing this.mdDialogRef.afterClosed but I think that the framework should ensure that cdk-global-scrollblock gets removed when navigating.

#### What is the use-case or motivation for changing an existing behavior?

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular: 4.3.0
Material: 2.0.0-beta.8-dd230a3
CDK: 2.0.0-beta.8-dd230a3
OS: Linux
Typescript: 2.3.4
Browsers: Chrome

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

Reproduisez le problème en ouvrant une boîte de dialogue, en appelant mdDialogRef.close() et en naviguant immédiatement vers une autre route. Suivez le cycle de vie de la fermeture de la boîte de dialogue et le nettoyage de cdk-global-scrollblock ; c’est terminé lorsque la classe est supprimée de document et que le défilement reste possible après la navigation.

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