Introduce infrastructure for tracing ScriptWrappables.

third_party/WebKit/Source/core/dom/Node.cpp

Index: third_party/WebKit/Source/core/dom/Node.cpp
diff --git a/third_party/WebKit/Source/core/dom/Node.cpp b/third_party/WebKit/Source/core/dom/Node.cpp
index a4045106859b7e4c826f9454f50db7776a577b3f..b870e48746b32d1ed7ebdfde1b74e37a7dab3379 100644
--- a/third_party/WebKit/Source/core/dom/Node.cpp
+++ b/third_party/WebKit/Source/core/dom/Node.cpp
@@ -339,6 +339,11 @@ NodeRareData* Node::rareData() const
     return static_cast<NodeRareData*>(m_data.m_rareData);
 }
 
+NodeRareData* Node::maybeRareData() const
+{
+    return hasRareData() ? rareData() : nullptr;
+}
+
 NodeRareData& Node::ensureRareData()
 {
     if (hasRareData())