Update the size expectation of sizeof(CSSValue)

Source/core/css/CSSValue.cpp

Index: Source/core/css/CSSValue.cpp
diff --git a/Source/core/css/CSSValue.cpp b/Source/core/css/CSSValue.cpp
index 01b1a71c680336341881da6526773e61f194faca..01289089a3173e5f60fdb23a362edb42f521bd27 100644
--- a/Source/core/css/CSSValue.cpp
+++ b/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;
 };