Oilpan: Build fix after r179102

It looks like r179102 increased sizeof(CSSValue) only in Windows builds with Oilpan enabled.
I don't know why. At the moment, this CL fixes the size expectation and fixed the compile error.

TBR=yukishiino@chromium.org

Review URL: https://codereview.chromium.org/427803003

git-svn-id: svn://svn.chromium.org/blink/trunk@179158 bbb929c8-8fbe-4397-9dbb-9b2b20218538