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
(cherry picked from commit 96a270589d2fa760cb5ef3df6c6868ceb1271913)