Remove 'webkit-backface-visibility' rule which made iPads super blurry

An extra CSS rule was snuck into the .transition mixin in I895679ff
which caused the entire content area in Vector skin to render blurry
on iPads, especially visible on non-Retina devices.

There was no mention of it in the commit message, but a comment in
the mixin source said something about a 'Chrome 1px movement bug'.

With no source for said Chrome bug and a definite Safari bug, let's
try removing it for now.

Bug: 67286
Change-Id: If6e88d2b5fa7adaa3cd8198686422a65689fa68b
Merge "Remove 'webkit-backface-visibility' rule which made iPads super blurry"