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

Start with the linked StackBlitz reproduction, especially app/select-reset-example.html, and inspect how cdk-virtual-scroll-viewport is nested in mat-select or mat-autocomplete. Reproduce the missing items and excess space while scrolling, then trace the overlay and virtual-scroll interaction. Done means newly scrolled values appear and the viewport aligns with the overlay panel.

Written by the indexing model from the issue text.