fix(fabToolbar): fix toolbar height to use variables
fix(fabToolbar): fix toolbar height to use variables

Fab Toolbar incorrectly used 6.8rem instead of the rem(6.8) SCSS function.
Also updated it to use button.scss variables instead of hard-coded values.

BREAKING CHANGE: md-fab-toolbar now uses variables for height

```css
md-fab-toolbar {
  height: $button-fab-width + ($icon-button-margin * 2);
}
```

closes #3384. closes #4007.

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