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

リンクされた StackBlitz の再現から始め、親要素に `zoom: 0.9` を指定した場合と指定しない場合の overlay の配置を比較します。CDK Overlay の位置指定のエントリポイントとその座標計算を追跡し、再現内で overlay がズームされていないコンテナに揃うことを確認します。

索引モデルが issue の本文から書いたものです。