Reland r179231 with oilpan build fixes

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

We enable CSSValueBaseAsEBCO only in MSVC because enabling CSSValueBaseAsEBCO in clang causes an error of the GC plugin. See discussion in https://codereview.chromium.org/429283002/ for more details.

Review URL: https://codereview.chromium.org/426333003

git-svn-id: svn://svn.chromium.org/blink/trunk@179402 bbb929c8-8fbe-4397-9dbb-9b2b20218538