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

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 ed9bb1256b950271dafaead45dfcaa11781408cb..4e65990c8b27929cf68ec205fec7339b8944230d 100644
--- a/Source/platform/heap/Visitor.h
+++ b/Source/platform/heap/Visitor.h
@@ -301,6 +301,12 @@
         OffHeapCollectionTraceTrait<Deque<T, N> >::trace(this, deque);
     }
 
+    template<typename T, typename U, typename V, typename W, typename X>
+    void trace(const HashMap<T, U, V, W, X, WTF::DefaultAllocator>& map)
+    {
+        OffHeapCollectionTraceTrait<HashMap<T, U, V, W, X, WTF::DefaultAllocator> >::trace(this, map);
+    }
+
 #if !ENABLE(OILPAN)
     // These trace methods are needed to allow compiling and calling trace on
     // transition types. We need to support calls in the non-oilpan build
@@ -430,6 +436,26 @@
     }
 };
 
+template<typename Key, typename Value, typename HashFunctions, typename KeyTraits, typename ValueTraits>
+struct OffHeapCollectionTraceTrait<WTF::HashMap<Key, Value, HashFunctions, KeyTraits, ValueTraits, WTF::DefaultAllocator> > {
+    typedef WTF::HashMap<Key, Value, HashFunctions, KeyTraits, ValueTraits, WTF::DefaultAllocator> HashMap;
+
+    static void trace(Visitor* visitor, const HashMap& map)
+    {
+        if (map.isEmpty())
+            return;
+        if (WTF::ShouldBeTraced<KeyTraits>::value || WTF::ShouldBeTraced<ValueTraits>::value) {
+            HashMap& iterMap = const_cast<HashMap&>(map);
+            for (typename HashMap::iterator it = iterMap.begin(), end = iterMap.end(); it != end; ++it) {
+                CollectionBackingTraceTrait<WTF::ShouldBeTraced<KeyTraits>::value, KeyTraits::weakHandlingFlag, WTF::WeakPointersActWeak, typename HashMap::KeyType, KeyTraits>::trace(visitor, it->key);
+                CollectionBackingTraceTrait<WTF::ShouldBeTraced<ValueTraits>::value, ValueTraits::weakHandlingFlag, WTF::WeakPointersActWeak, typename HashMap::MappedType, ValueTraits>::trace(visitor, it->value);
+            }
+        }
+        COMPILE_ASSERT(KeyTraits::weakHandlingFlag == WTF::NoWeakHandlingInCollections, WeakOffHeapCollectionsConsideredDangerous1);
+        COMPILE_ASSERT(ValueTraits::weakHandlingFlag == WTF::NoWeakHandlingInCollections, WeakOffHeapCollectionsConsideredDangerous2);
+    }
+};
+
 // We trace vectors by using the trace trait on each element, which means you
 // can have vectors of general objects (not just pointers to objects) that can
 // be traced.