mat-form-field does not remove mat-form-field-invalid class on FormGroup reset

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

#### What is the expected behavior?
Upon resetting a FormGroup, I would expect the visual state of the form to be reset also.

#### What is the current behavior?
The form group and controls are reset but the mat-input-invalid class remains on the md-input-container suggesting that the control is invalid.

#### What are the steps to reproduce?
This only seems to happen when the [formGroup] attribute is on a form tag. Works as expected on a div tag.

Create component with a FormGroup and a FormControl with Validators.required .
Create a form tag with the [formGroup] attribute.
Add an input control to the form with a required attribute.

When running, fill out the form so its valid, then reset the form and model through the click handler on a button. The required control will now have the mat-input-invalid class.

#### Which versions of Angular, Material, OS, browsers are affected?
Angular 4.0.1
Material 2.0.0-beta.3
Windows 10
Chrome 57

首先，使用表单元素上的 Angular FormGroup 和必填 FormControl 重现该问题，然后将其与相同控件位于 div 上的情况进行比较。通过相关的 Angular Material 源代码跟踪表单重置和 mat-form-field 验证状态；当重置 group 移除 invalid 类并为两个容器保留预期行为时，即表示完成。

由索引模型根据 Issue 内容生成。