virtual-scroll inside mat-select new values not visible & big space

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

Bug

#### What is the expected behavior?

`cdk-virtual-scroll-viewport` inside a mat-select or mat-autocomplete should align with the overlay panel.

```

{{state}}

```

#### What is the current behavior?

When scrolled `cdkVirtualFor` does not seem to add more elements and moreover shows a big space from last element of the initial set of items to the end.

#### What are the steps to reproduce?

https://stackblitz.com/edit/angular-h4xptu-dgjd87?file=app/select-reset-example.html

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

* Occasionally I run into scenarios where the items in the list are too many and makes the page unresponsive.

* Seem-less integration of cdk-virtual-scroll-viewport with other components.

* mat-select essentially can handle any number of elements when this works.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?

```
Angular CLI: 6.1.5
Node: 10.0.0
OS: win32 x64
Angular: 6.1.4
... animations, common, compiler, core, forms, http
... platform-browser, platform-browser-dynamic, router

Package Version
------------------------------------------------------------
@angular-devkit/architect 0.7.5
@angular-devkit/build-angular 0.7.5
@angular-devkit/build-ng-packagr 0.7.5
@angular-devkit/build-optimizer 0.7.5
@angular-devkit/build-webpack 0.7.5
@angular-devkit/core 0.7.5
@angular-devkit/schematics 0.7.5
@angular/cdk 6.4.7
@angular/cdk-experimental 6.4.7
@angular/cli 6.1.5
@angular/compiler-cli 6.1.7
@angular/flex-layout 6.0.0-beta.17
@angular/language-service 6.1.7
@angular/material 6.4.7
@ngtools/webpack 6.1.5
@schematics/angular 0.7.5
@schematics/update 0.7.5
rxjs 6.2.2
typescript 2.9.2
webpack 4.9.2
```

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

Great work

Beginnen Sie mit der verknüpften StackBlitz-Reproduktion, insbesondere mit app/select-reset-example.html, und untersuchen Sie, wie cdk-virtual-scroll-viewport in mat-select oder mat-autocomplete verschachtelt ist. Reproduzieren Sie die fehlenden Elemente und den zusätzlichen Leerraum beim Scrollen und verfolgen Sie anschließend die Interaktion zwischen Overlay und Virtual Scroll. Als erledigt gilt die Aufgabe, wenn neu gescrollte Werte erscheinen und der Viewport am Overlay-Panel ausgerichtet ist.

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