Oilpan: Make sure that vtables for garbage collected mixin objects have

Source/platform/heap/Visitor.h

Index: Source/platform/heap/Visitor.h
diff --git a/Source/platform/heap/Visitor.h b/Source/platform/heap/Visitor.h
index 73a6d5511aeee34aa0a2dc094fccdefb7bd63d97..f9dffd5ba41ed4aa4411fef73c7e58141b59cdc6 100644
--- a/Source/platform/heap/Visitor.h
+++ b/Source/platform/heap/Visitor.h
@@ -586,8 +586,9 @@ template<typename T> bool ObjectAliveTrait<T>::isAlive(Visitor* visitor, T* obj)
 
 class GarbageCollectedMixin {
 public:
-    virtual void adjustAndMark(Visitor*) const = 0;
-    virtual bool isAlive(Visitor*) const = 0;
+    virtual void adjustAndMark(Visitor*) const { };

[zerny-chromium, 2014/07/19 07:21:58]

DBC we need to extend the plugin checks (or find a suitable static assert) after
this change since we relied on the pure virtual compiler checks to catch the
error if a mixin did not use the USING... macro.

+    virtual bool isAlive(Visitor*) const { return true; };
+    virtual void trace(Visitor*) { }
 };
 
 #define USING_GARBAGE_COLLECTED_MIXIN(TYPE) \