Feeling of content scaling and fading in.

scale-fade-up will move in from a far plane.

• Scale from .6-ish to 1.
• Scale and opacity from 0 to 1.

scale-fade-up will move in from a near plane.

• Scale from 1.3-ish to 1.
• opacity from 0 to 1.

cc @ampproject/wg-stories @hongwei1990