Remove CSS Variables

This patch removes the current CSS Variables implementation inherited
from WebKit.

Our CSS Variables implementation in its current state needs a rewrite
before it is ready to ship. Our Bison CSS Parser is slated to be
rewritten from scratch, this would result in another rewrite of the
variables implementation. CSS Variables should be removed for the time
being to prevent bitrot.


BUG=342667

Review URL: https://codereview.chromium.org/158043005

git-svn-id: svn://svn.chromium.org/blink/trunk@166920 bbb929c8-8fbe-4397-9dbb-9b2b20218538
DevTools: Unskip a test that does not fail anymore.

BUG=334242
TBR=alph

Review URL: https://codereview.chromium.org/146143006

git-svn-id: svn://svn.chromium.org/blink/trunk@166921 bbb929c8-8fbe-4397-9dbb-9b2b20218538