Autocomplete initial positioning

#### Bug, feature request, or proposal:
I have a project where the `autocomplete` dropdown does not properly position itself until it the input box that it is acting upon is unfocused and refocused. It initially extends past the bottom of the screen, choosing to initially position on the bottom of its input box.

Perhaps there could be a directive to force positioning of an `autocomplete` dropdown?

#### What is the expected behavior?
Expected behavior is for the `autocomplete` dropdown to recognize that it will be off-screen if it extends downward and instead position itself above the input box.

#### What is the current behavior?
In its current behavior, the `autocomplete` dropdown only calculates its position properly once its input box has been refocused. Switching tabs also causes the `autocomplete` dropdown to position properly.

#### What are the steps to reproduce?
The project in which this bug is occurring is already large and mature. I have, as of yet, been unable to reproduce this issue within a simple context.

#### What is the use-case or motivation for changing an existing behavior?
There may be cases where the code does not properly position the `autocomplete` dropdown. Adding a directive or other such mechanism for forcing positioning may prove useful.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
```
@angular/cli: 1.5.0
@angular/material: 2.0.0-beta.12
typescript: 2.3.3
OS: (Gentoo) Linux
Browsers: webkit and gecko
```

#### Is there anything else we should know?
I don't particularly expect great support since I am unable to provide reproducable steps, but thanks in advance anybody's time in addressing this matter.

ソースファイル、テスト、再現可能な例のいずれも指定されていません。まず、viewport の下端付近で autocomplete が開く最小限の再現を作成し、次に初期配置の経路を追跡して、refocus およびタブ切り替え時の挙動と比較してください。画面外にはみ出す場合に、最初に dropdown が入力欄の上へ移動すれば完了です。

索引モデルが issue の本文から書いたものです。