Update the size expectation of sizeof(CSSValue)

This is a follow-up CL for r179158.
Applied tkent-san's comment in https://codereview.chromium.org/427803003/#msg4.

Review URL: https://codereview.chromium.org/423293002

git-svn-id: svn://svn.chromium.org/blink/trunk@179189 bbb929c8-8fbe-4397-9dbb-9b2b20218538