Progress spinner shakes when specifying a diameter.

#### Bug:

When specifying a diameter (16-32px) on the progress spinner the animation will eventually start to shake by a couple pixels as it animates. Using smaller diameters make it more noticeable. Using other components with animations (material inputs, etc.) makes it worse.

#### What is the expected behavior?

Progress spinner has a smooth animation that doesn't shake as it animates.

#### What is the current behavior?

Over time, the progress spinner will begin to shake up and down as it animates.

#### What are the steps to reproduce?

https://stackblitz.com/edit/angular-material2-issue-xg5xgq

1) Load the stackblitz demo
3) Click on the input (trigger the label animation)
4) Click away from the input (return it to normal)
5) Watch for the progress spinner to shake.
6) The progress spinner animation will never recover.

#### What is the use-case or motivation for changing an existing behavior?

The issue appears worse the more material components are on the page. Currently, progress spinner is unusable in our app because the shaking is distracting and unpleasant.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Angular: 6.0.3
Material: 6.2.0
Chrome: Version 67.0.3396.79 (Official Build) (64-bit)

リンクされた StackBlitz の再現から始め、progress-spinner の実装を調査し、input label がトリガーされた後の直径固有のアニメーションに焦点を当ててください。影響を受ける設定で shake を確認し、input にフォーカスを当ててから外した後も 16–32px の spinner が滑らかなままであることを確認してください。

索引モデルが issue の本文から書いたものです。