Oilpan: keep navigation timing objects on the heap by default.

Source/core/timing/Performance.h

Index: Source/core/timing/Performance.h
diff --git a/Source/core/timing/Performance.h b/Source/core/timing/Performance.h
index a201b545a7e158bba93ebc17cefd6dec15b1a56f..de4f1db8f21ed5d514d253e906107ba02a1b50db 100644
--- a/Source/core/timing/Performance.h
+++ b/Source/core/timing/Performance.h
@@ -97,8 +97,8 @@ private:
     bool isResourceTimingBufferFull();
     void addResourceTimingBuffer(PerformanceEntry*);
 
-    mutable RefPtrWillBeMember<PerformanceNavigation> m_navigation;
-    mutable RefPtrWillBeMember<PerformanceTiming> m_timing;
+    mutable Member<PerformanceNavigation> m_navigation;
+    mutable Member<PerformanceTiming> m_timing;
 
     PerformanceEntryVector m_resourceTimingBuffer;
     unsigned m_resourceTimingBufferSize;