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

Source/modules/indexeddb/IDBPendingTransactionMonitor.h

Index: Source/modules/indexeddb/IDBPendingTransactionMonitor.h
diff --git a/Source/modules/indexeddb/IDBPendingTransactionMonitor.h b/Source/modules/indexeddb/IDBPendingTransactionMonitor.h
index 6d4b4d94d64921433831e8989b04d0d862c28ab6..3eff46e34a366959e3ab9ce1fcaacbf97a2ac301 100644
--- a/Source/modules/indexeddb/IDBPendingTransactionMonitor.h
+++ b/Source/modules/indexeddb/IDBPendingTransactionMonitor.h
@@ -41,14 +41,15 @@
 // which is set to true on creation, but must be set to false when control
 // returns to the event loop.
 
-class IDBPendingTransactionMonitor : public NoBaseWillBeGarbageCollected<IDBPendingTransactionMonitor>, public WillBeHeapSupplement<ExecutionContext> {
-    WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(IDBPendingTransactionMonitor);
-    DECLARE_EMPTY_VIRTUAL_DESTRUCTOR_WILL_BE_REMOVED(IDBPendingTransactionMonitor);
+class IDBPendingTransactionMonitor : public Supplement<ExecutionContext> {
     WTF_MAKE_NONCOPYABLE(IDBPendingTransactionMonitor);
 
 public:
-    static IDBPendingTransactionMonitor& from(WillBeHeapSupplementable<ExecutionContext>&);
-    virtual void trace(Visitor*) OVERRIDE;
+    static IDBPendingTransactionMonitor& from(Supplementable<ExecutionContext>&);
+    virtual ~IDBPendingTransactionMonitor();
+    // The trace functino doesn't work until ExecutionContext is moved to Oilpan
+    // heap.
+    virtual void trace(Visitor* visitor) OVERRIDE { Supplement<ExecutionContext>::trace(visitor); }
     void addNewTransaction(IDBTransaction&);
     void deactivateNewTransactions();
 
@@ -56,7 +57,8 @@
     IDBPendingTransactionMonitor();
     static const char* supplementName();
 
-    PersistentHeapVectorWillBeHeapVector<Member<IDBTransaction> > m_transactions;
+    typedef PersistentHeapVector<Member<IDBTransaction> > TransactionList;
+    TransactionList m_transactions;
 };
 
 } // namespace blink