Oilpan: Build fix after r179102

It looks like r179102 increased sizeof(CSSValue) only in Windows builds with Oilpan enabled.
I don't know why. At the moment, this CL fixes the size expectation and fixed the compile error.

TBR=yukishiino@chromium.org

Review URL: https://codereview.chromium.org/427803003

git-svn-id: svn://svn.chromium.org/blink/trunk@179158 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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

@@ -58,7 +58,7 @@
 
 namespace blink {
 
-struct SameSizeAsCSSValue : public RefCountedWillBeGarbageCollectedFinalized<SameSizeAsCSSValue> {
+struct SameSizeAsCSSValue : public RefCountedWillBeGarbageCollectedFinalized<SameSizeAsCSSValue>, public ScriptWrappableBase {
     uint32_t bitfields;
 };