bindings: Workaround of missing EBCO of VC++.

VC++ doesn't support Empty Base Class Optimization.  Adds a workaround to avoid the bloat of instance size of CSSValue.  For details see the comment in CSSValue.h.

Review URL: https://codereview.chromium.org/423383003

git-svn-id: svn://svn.chromium.org/blink/trunk@179231 bbb929c8-8fbe-4397-9dbb-9b2b20218538