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)

Comienza con la reproducción vinculada de StackBlitz e inspecciona la implementación de progress-spinner, centrándote en la animación específica del diámetro después de que se active la etiqueta de entrada. Confirma el shake en la configuración afectada y verifica que un spinner de 16–32px siga funcionando con fluidez después de que el campo de entrada reciba y pierda el foco.

Escrito por el modelo de indexación a partir del texto del issue.