Oilpan: trace non-owned InspectorOverlay references.

Source/core/inspector/InspectorDOMAgent.cpp

Index: Source/core/inspector/InspectorDOMAgent.cpp
diff --git a/Source/core/inspector/InspectorDOMAgent.cpp b/Source/core/inspector/InspectorDOMAgent.cpp
index f35e5a5e68d78e3407f1c215a1b5649d8889f9c6..e6227152930e3e3fb704ded97343df2b977f2e9e 100644
--- a/Source/core/inspector/InspectorDOMAgent.cpp
+++ b/Source/core/inspector/InspectorDOMAgent.cpp
@@ -2356,6 +2356,7 @@ DEFINE_TRACE(InspectorDOMAgent)
     visitor->trace(m_domListener);
     visitor->trace(m_pageAgent);
     visitor->trace(m_injectedScriptManager);
+    visitor->trace(m_overlay);
 #if ENABLE(OILPAN)
     visitor->trace(m_documentNodeToIdMap);
     visitor->trace(m_danglingNodeToIdMaps);