[Textarea] Scroll position changes unexpectedly

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

#### What is the expected behavior?
When filling a textarea, we should see what we are writing :-)

#### What is the current behavior?
Under some conditions, writing inside a textarea with autoSize directive makes the page scroll position changes (we get scrolled up ? sorry for my english).

#### What are the steps to reproduce?
To reproduce my issue:
- With latest firefox (tested on macOS), open https://angular-material2-issue-pnr2sj.stackblitz.io/
- Scroll down the page.
- Enter the textarea and press any key.
- Your scroll position changes on the page, most of the textarea content isn't visible anymore, which is very annoying for writing.

It also happens on iOS 11 when the textarea is inside a parent with scroll overflow (not the case on the stackblitz example, but I can create another one if needed). I cannot reproduce it on Chrome and I haven't tested other browsers.

I believe the issue is related to [this line](https://github.com/angular/material2/blob/master/src/lib/input/autosize.ts#L197), the parent's height get shorter for an instant which explains we get scrolled up.

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

Inizia da src/lib/input/autosize.ts intorno alla riga 197 e riproduci il problema usando l'esempio StackBlitz collegato in Firefox, quindi confronta il comportamento quando la textarea si trova all'interno di un elemento padre con scroll overflow su iOS. Il lavoro è completato quando la digitazione in una textarea autoSize non modifica più la posizione di scorrimento della pagina o dell'elemento padre e il contenuto inserito rimane visibile.

Scritto dal modello di indicizzazione a partire dal testo della issue.