mat-icon: To be able to have both svgIcon and fontIcon

#### Please describe the feature you would like to request.
Let's say you have a js structure (an array of object) holding a list of icons, some svg, some font with other properties.
In your view you want to have a simple *ngFor to iterate on all your icons.
For svg icons, your object will have a "svg" property.
For font icons, your object will have a "font" property.
The key will be undefined if not set.
Therefore, you will have in your loop something like:
`{{ btn.icon }}`
Sounds pretty easy but that's not working right now (Angular Material 7.3.7).

#### What is the use-case or motivation for this proposal?
To simply KISS
To avoid a fugly:
``
`{{ btn.icon }}`

#### Is there anything else we should know?
For what I saw, the reason about that is located in:
https://github.com/angular/material2/blob/master/src/lib/icon/icon.ts#L224-L244

A quick hack could be :
https://github.com/angular/material2/blob/master/src/lib/icon/icon.ts#L237
`!this._usingFontIcon() && this._clearSvgElement();`

从 src/lib/icon/icon.ts 的第 224-244 行开始，使用报告中描述的 Angular Material 7.3.7 mat-icon/ngFor 示例复现该问题。跟踪 svgIcon 和 fontIcon 的处理过程，然后验证单个 mat-icon 可以在不使用条件模板的情况下渲染任一种形式，同时保留现有行为。

由索引模型根据 Issue 内容生成。