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.

未指定源文件、测试或可复现示例。首先构建一个在 viewport 底部附近打开 autocomplete 的最小复现，然后跟踪初始定位路径，并将其与重新聚焦和切换标签页时的行为进行比较。当 dropdown 在会超出屏幕范围时初始移动到输入框上方，即表示完成。

由索引模型根据 Issue 内容生成。