Remove all usage of -webkit-*gradient inside blink

The prefixed functions have been deprecated in 2011 and should
be use counted with the goal of removing them.

This patch just rewrites existing rules on top of the modern syntax:

-webkit-gradient(linear, top left, bottom left, from(white), to(black))

is equivalent to

linear-gradient(to bottom, white, black)

Review URL: https://codereview.chromium.org/152493003

git-svn-id: svn://svn.chromium.org/blink/trunk@166426 bbb929c8-8fbe-4397-9dbb-9b2b20218538