Fix PersistentHeapHashMap to work with stack allocated, heap backed weak collections

Source/wtf/HashTable.h

Index: Source/wtf/HashTable.h
diff --git a/Source/wtf/HashTable.h b/Source/wtf/HashTable.h
index 30c8bbd5ef55f94a0169bbea561c132d835d5fcd..02c24bb5a3e70a69993230619c3149ccd5d0d80f 100644
--- a/Source/wtf/HashTable.h
+++ b/Source/wtf/HashTable.h
@@ -1071,7 +1071,7 @@ namespace WTF {
         if (!Traits::isWeak)
             Allocator::markNoTracing(visitor, m_table);
         else
-            Allocator::registerWeakMembers(visitor, this, WeakProcessingHashTableHelper<Traits::isWeak, Key, Value, Extractor, HashFunctions, Traits, KeyTraits, Allocator>::process);
+            Allocator::registerWeakMembers(visitor, this, m_table, WeakProcessingHashTableHelper<Traits::isWeak, Key, Value, Extractor, HashFunctions, Traits, KeyTraits, Allocator>::process);

[sof, 2014/03/12 15:48:15]

Can m_table be 0 ?

[Mads Ager (chromium), 2014/03/12 15:53:50]

Good catch! It most definitely can. In that case we don't need a weak callback,
so we just need to not register weak members if m_table is null.

         if (ShouldBeTraced<Traits>::value) {
             for (ValueType* element = m_table + m_tableSize - 1; element >= m_table; element--) {
                 if (!isEmptyOrDeletedBucket(*element))