Oilpan: fix build after r181867.

Source/core/inspector/InspectorTraceEvents.cpp

Index: Source/core/inspector/InspectorTraceEvents.cpp
diff --git a/Source/core/inspector/InspectorTraceEvents.cpp b/Source/core/inspector/InspectorTraceEvents.cpp
index 62c174d918812673a9622b48f6ec05168a14b9b2..6545c370331233cb7728ee7ecb0271fbb40e592e 100644
--- a/Source/core/inspector/InspectorTraceEvents.cpp
+++ b/Source/core/inspector/InspectorTraceEvents.cpp
@@ -122,7 +122,7 @@ PassRefPtr<TraceEvent::ConvertableToTraceFormat> InspectorLayoutInvalidationTrac
         value->setString("nodeName", generatingNode->debugName());
         value->setInteger("nodeId", InspectorNodeIds::idForNode(generatingNode));
     }
-    RefPtr<ScriptCallStack> callstack = createScriptCallStack(maxInvalidationTrackingCallstackSize, true);
+    RefPtrWillBeRawPtr<ScriptCallStack> callstack = createScriptCallStack(maxInvalidationTrackingCallstackSize, true);
     value->setString("callstack", callstack ? callstack->buildInspectorArray()->toJSONString() : "[]");
     return value;
 }