Oilpan: Ensure that classes with virtual trace methods always have vtables for their left-most base…

Source/platform/LifecycleContext.h

Index: Source/platform/LifecycleContext.h
diff --git a/Source/platform/LifecycleContext.h b/Source/platform/LifecycleContext.h
index c7f1ae0af5f0aeac633bb590bbd864807d5b8ad5..d2eee95e45287eb6a63c2a09c35b9b384d67f8ea 100644
--- a/Source/platform/LifecycleContext.h
+++ b/Source/platform/LifecycleContext.h
@@ -36,6 +36,8 @@
 
 namespace blink {
 
+class Visitor;
+
 template <typename T>
 class LifecycleContext {
 public:
@@ -53,6 +55,8 @@ public:
     // Called from the destructor of observers.
     void wasUnobservedBy(Observer*);
 
+    virtual void trace(Visitor*) { }
+
 protected:
     Notifier& lifecycleNotifier();