Oilpan: trace non-owned InspectorOverlay references.

Source/core/inspector/InspectorPageAgent.cpp

Index: Source/core/inspector/InspectorPageAgent.cpp
diff --git a/Source/core/inspector/InspectorPageAgent.cpp b/Source/core/inspector/InspectorPageAgent.cpp
index ae8a944af716944948d314a0c443a8356b14a073..9db25cd3fa11e5096b2372c8dfeeedf784974b87 100644
--- a/Source/core/inspector/InspectorPageAgent.cpp
+++ b/Source/core/inspector/InspectorPageAgent.cpp
@@ -1375,9 +1375,9 @@ DEFINE_TRACE(InspectorPageAgent)
     visitor->trace(m_injectedScriptManager);
     visitor->trace(m_debuggerAgent);
     visitor->trace(m_cssAgent);
+    visitor->trace(m_overlay);
     visitor->trace(m_inspectorResourceContentLoader);
     InspectorBaseAgent::trace(visitor);
 }
 
 } // namespace blink
-