Define a bounding box for overlay other than documentElement

#### Please describe the feature you would like to request.
I would like the ability to define a bounding area where the tooltip is constrained to. Currently, the overlay bounding box is set to top: 0px; left: 0px; height: 100%; width: 100% but I would like to define these programmatically or pass an element ref to use as a template for the bounding box calculation. I know I can add a class to the overlay to change this but it has to be dynamic and respond to resizing an element whose size represents what the overlay bounds should be.

#### What is the use-case or motivation for this proposal?
The app has a requirement to restrict the tooltip location over a certain area and not allow it to overflow over surrounding elements. For example, using a tooltip to provide more detail over a chart cursor but the tooltip should be constrained to just the chart area.

This isn't the exact scenario but you can see the tooltip overflows the mat-card area here. I want to be able to push the tooltip into the mat-card area.
https://stackblitz.com/edit/angular6-material-components-demo-overlay-container

#### Is there anything else we should know?
Angular 7.2.4
Material 7.2.1

In FlexibleConnectedPositionStrategy, perhaps there could be an optional sizingEl property to use other than the documentElement. I haven't been able to test this since it's not extendable.

```
withSizingElement(sizingElement: HTMLElement): this {
this._sizingEl = sizingElement;
return this;
}

private _getNarrowedViewportRect(): ClientRect {
const el = this._sizingEl || this._document.documentElement;
const width = el!.clientWidth;
const height = el!.clientHeight;
...
}
```
Something similar will have to be done to get the top and left from the sizing element:
```
getScrollPosition(referenceElement?: HTMLElement): ScrollPosition {
if (!this._platform.isBrowser) {
return {top: 0, left: 0};
}

const el = referenceElement || document.documentElement!;
const rect = el.getBoundingClientRect();
const top = -rect.top || el.scrollTop || 0;
const left = -rect.left || el.scrollLeft || 0;

return {top, left};
}
```

Commencez par lire FlexibleConnectedPositionStrategy, en particulier _getNarrowedViewportRect et getScrollPosition, afin de comprendre comment les limites actuelles de documentElement sont calculées. Définissez le comportement pour un sizing element fourni, notamment sa position et son redimensionnement, et vérifiez que les tooltips restent contraints à la zone de chart ou de mat-card demandée.

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