image squeezed on hide for <app-header reveals effects="parallax-background">

[valdrinkoshi]

The hide animation of app-header squeezes the background image before the animation is done.

To reproduce:

• navigate to https://polymerelements.github.io/app-layout/templates/test-drive/
• enable only reveals property and parallax-background effect
• scroll down enough to make the header disappear
• scroll up a bit so that the header appears again
• start scroll down, notice the image distortion