Make ComputedStyleBase use MemberCopy when copying pointer fields.

third_party/WebKit/Source/core/style/MemberCopy.h

Index: third_party/WebKit/Source/core/style/MemberCopy.h
diff --git a/third_party/WebKit/Source/core/style/MemberCopy.h b/third_party/WebKit/Source/core/style/MemberCopy.h
new file mode 100644
index 0000000000000000000000000000000000000000..93631e8109743eb38fe64c4a4fe431ed3c8db519
--- /dev/null
+++ b/third_party/WebKit/Source/core/style/MemberCopy.h
@@ -0,0 +1,25 @@
+// Copyright 2017 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.
+
+#ifndef MemberCopy_h
+#define MemberCopy_h
+
+#include "platform/heap/Persistent.h"
+#include "platform/wtf/RefPtr.h"
+
+namespace blink {
+
+template <typename T>
+RefPtr<T> MemberCopy(const RefPtr<T>& v) {
+  return v;
+}
+
+template <typename T>
+Persistent<T> MemberCopy(const Persistent<T>& v) {
+  return v;
+}
+
+}  // namespace blink
+
+#endif  // MemberCopy_h