Add expansion of shorthands with custom properties to longhands using a pending substition value.

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

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "core/css/CSSPendingSubstitutionValue.h"
+
+namespace blink {
+
+DEFINE_TRACE_AFTER_DISPATCH(CSSPendingSubstitutionValue)
+{
+    CSSValue::traceAfterDispatch(visitor);
+    visitor->trace(m_shorthandValue);
+}
+
+String CSSPendingSubstitutionValue::customCSSText() const
+{
+    return "";
+}
+
+} // namespace blink