Oilpan: trace non-owned InspectorOverlay references.

Source/core/inspector/PageDebuggerAgent.cpp

Index: Source/core/inspector/PageDebuggerAgent.cpp
diff --git a/Source/core/inspector/PageDebuggerAgent.cpp b/Source/core/inspector/PageDebuggerAgent.cpp
index c9a4703c7d5c45ce3ebea142e2b2aa72178aab43..e877c4c99f608d738fdbef446528f9e89a660061 100644
--- a/Source/core/inspector/PageDebuggerAgent.cpp
+++ b/Source/core/inspector/PageDebuggerAgent.cpp
@@ -67,6 +67,7 @@ DEFINE_TRACE(PageDebuggerAgent)
 {
     visitor->trace(m_pageScriptDebugServer);
     visitor->trace(m_pageAgent);
+    visitor->trace(m_overlay);
     InspectorDebuggerAgent::trace(visitor);
 }
 
@@ -151,4 +152,3 @@ void PageDebuggerAgent::didClearDocumentOfWindowObject(LocalFrame* frame)
 }
 
 } // namespace blink
-