Oilpan: Prepare moving InspectorOverlayHost to Oilpan

Source/core/inspector/InspectorOverlayHost.cpp

Index: Source/core/inspector/InspectorOverlayHost.cpp
diff --git a/Source/core/inspector/InspectorOverlayHost.cpp b/Source/core/inspector/InspectorOverlayHost.cpp
index ba6025603917f66959c2db9abb852005cd037bdb..5d0784e7fd5c9cdb396fe841228812a0bd850783 100644
--- a/Source/core/inspector/InspectorOverlayHost.cpp
+++ b/Source/core/inspector/InspectorOverlayHost.cpp
@@ -33,7 +33,7 @@
 namespace blink {
 
 InspectorOverlayHost::InspectorOverlayHost()
-    : m_listener(0)
+    : m_listener(nullptr)
 {
     ScriptWrappable::init(this);
 }
@@ -54,4 +54,9 @@ void InspectorOverlayHost::stepOver()
         m_listener->overlaySteppedOver();
 }
 
+void InspectorOverlayHost::trace(Visitor* visitor)
+{
+    visitor->trace(m_listener);
+}
+
 } // namespace blink