Build fix after r179402

r179402 tried to fix the assertion about sizeof(CSSValue) in oilpan-enabled MSVC,
but it didn't fix it. At the moment, this CL updates the size expectation to fix the compile error.

TBR=yukishiino@chromium.org

Review URL: https://codereview.chromium.org/437803004

git-svn-id: svn://svn.chromium.org/blink/trunk@179420 bbb929c8-8fbe-4397-9dbb-9b2b20218538