Safari: Bounded ripples are no longer masked by surface's border-radius

[kfranqueiro]

What MDC-Web Version are you using?

0.27.0

What browser(s) is this bug affecting?

OS X Safari

What are the steps to reproduce the bug?

Easiest to see in fab.html when you hover or click on a FAB

What is the expected behavior?

Overlay should only be visible within the FAB's rounded borders

What is the actual behavior?

Overlay occupies entire square region around FAB

Any other information you believe would be useful?

Andy noticed that adding a z-index or a translateZ transform to the ripple surface element fixes this. I suspect this may have broken when will-change was moved to the before/after pseudo-elements (which ended up resolving #1665, so we need to make sure not to cause regressions with that when we fix this).