Clear old backing store of WeakCollection on updates.

src/runtime/runtime-collections.cc

Index: src/runtime/runtime-collections.cc
diff --git a/src/runtime/runtime-collections.cc b/src/runtime/runtime-collections.cc
index c4f74183ce4a617f0bff2d5dfebd05812911f395..c1a63dc8b4373b115288801773479f8a3d1c9f75 100644
--- a/src/runtime/runtime-collections.cc
+++ b/src/runtime/runtime-collections.cc
@@ -288,6 +288,10 @@ RUNTIME_FUNCTION(Runtime_WeakCollectionDelete) {
   Handle<ObjectHashTable> new_table =
       ObjectHashTable::Remove(table, key, &was_present);
   weak_collection->set_table(*new_table);
+  if (*table != *new_table) {
+    // Zap the old table since we didn't record slots for its elements.
+    table->FillWithHoles(0, table->length());
+  }
   return isolate->heap()->ToBoolean(was_present);
 }
 
@@ -304,6 +308,10 @@ RUNTIME_FUNCTION(Runtime_WeakCollectionSet) {
   RUNTIME_ASSERT(table->IsKey(*key));
   Handle<ObjectHashTable> new_table = ObjectHashTable::Put(table, key, value);
   weak_collection->set_table(*new_table);
+  if (*table != *new_table) {
+    // Zap the old table since we didn't record slots for its elements.
+    table->FillWithHoles(0, table->length());
+  }
   return *weak_collection;
 }