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

third_party/WebKit/Source/core/css/CSSValue.cpp

@@ -58,7 +58,13 @@
 
 namespace blink {
 
-struct SameSizeAsCSSValue : public RefCountedWillBeGarbageCollectedFinalized<SameSizeAsCSSValue>, public ScriptWrappableBase {
+struct SameSizeAsCSSValue : public RefCountedWillBeGarbageCollectedFinalized<SameSizeAsCSSValue>
+// FIXME: Figure out why only win builds with oilpan increase sizeof(CSSValue).
+// Deriving ScriptWrappableBase should not increase sizeof(CSSValue).
+#if ENABLE(OILPAN) && OS(WIN)
+    , public ScriptWrappableBase
+#endif
+{
     uint32_t bitfields;
 };