CDK Overlay - Position Strategy with zoom css on parent

#### Bug, feature request, or proposal:
If you are using the `zoom` CSS property to scale down UI elements the positioning of the CDK Overlay are calculated incorrectly.

#### What is the expected behavior?
Overlay positions properly as the non-zoomed styled `div`

#### What is the current behavior?
Overlay positions are calculated incorrectly when the `zoom: 0.9` css property is on the parent container.

#### What are the steps to reproduce?
https://stackblitz.com/edit/cdk-overlay-zoom

#### What is the use-case or motivation for changing an existing behavior?
We have a Chrome only application that we use the `zoom` css property to scale down the UI. Using other methods of scaling does not work, so this is what we have to do.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular - 5.2.10
CDK - 5.2.5

Commencez par la reproduction StackBlitz liée et comparez le positionnement de l’overlay avec et sans `zoom: 0.9` sur l’élément parent. Suivez le point d’entrée du positionnement de CDK Overlay et ses calculs de coordonnées, puis vérifiez que l’overlay s’aligne sur le conteneur sans zoom dans la reproduction.

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