[oilpan]: Remove support for tracing off-heap hashmaps.

Source/platform/Supplementable.h

Index: Source/platform/Supplementable.h
diff --git a/Source/platform/Supplementable.h b/Source/platform/Supplementable.h
index f148734e003f11811ce5de2f76adb71f20dcf6bb..0465f9e2dedd6d0e5faa5aab853c0e5a8e29790d 100644
--- a/Source/platform/Supplementable.h
+++ b/Source/platform/Supplementable.h
@@ -188,8 +188,6 @@ public:
 #endif
     }
 
-    virtual void trace(Visitor* visitor) { visitor->trace(m_supplements); }
-
     void willBeDestroyed()
     {
         typedef typename SupplementableTraits<T, isGarbageCollected>::SupplementMap::iterator SupplementIterator;
@@ -216,9 +214,9 @@ class HeapSupplement : public SupplementBase<T, true> { };
 
 // FIXME: Oilpan: Move GarbageCollectedMixin to SupplementableBase<T, true> once PersistentHeapSupplementable is removed again.
 template<typename T>
-class GC_PLUGIN_IGNORE("http://crbug.com/395036") HeapSupplementable : public SupplementableBase<T, true>, public GarbageCollectedMixin {
+class HeapSupplementable : public SupplementableBase<T, true>, public GarbageCollectedMixin {
 public:
-    virtual void trace(Visitor* visitor) { SupplementableBase<T, true>::trace(visitor); }
+    virtual void trace(Visitor* visitor) OVERRIDE { visitor->trace(this->m_supplements); }
 };
 
 template<typename T>
@@ -231,6 +229,9 @@ public:
         for (SupplementIterator it = this->m_supplements.begin(); it != this->m_supplements.end(); ++it)
             it->value->persistentHostHasBeenDestroyed();
     }
+
+    virtual void trace(Visitor* visitor) { visitor->trace(this->m_supplements); }
+
 private:
     class TraceDelegate : PersistentBase<ThreadLocalPersistents<AnyThread>, TraceDelegate> {
     public:
@@ -247,7 +248,16 @@ template<typename T>
 class Supplement : public SupplementBase<T, false> { };
 
 template<typename T>
-class Supplementable : public SupplementableBase<T, false> { };
+class Supplementable : public SupplementableBase<T, false> {
+public:
+    virtual void trace(Visitor*)
+    {
+        // No tracing of off-heap supplements. We should not have any Supplementable

[sof, 2014/07/21 13:43:15]

I think it is unlikely that this slips through, but worth strengthening with an
assert of some kind?

[wibling-chromium, 2014/07/21 14:37:43]

Good idea. I added a compile assert checking that T is not garbage collected
when this trace method is instantiated.

+        // object on the heap. Either the object is HeapSupplementable or if it is
+        // off heap it should use PersistentHeapSupplementable to trace any on-heap
+        // supplements.
+    }
+};
 
 template<typename T>
 struct ThreadingTrait<blink::SupplementBase<T, true> > {