Rehash and clear deleted entries in weak collections during GC

src/heap/mark-compact.h

Index: src/heap/mark-compact.h
diff --git a/src/heap/mark-compact.h b/src/heap/mark-compact.h
index a0f54acac7dea26f9a34f642ef4bcf14cd6e8610..591cf183ab7bc88416a0699707609d1c7cf43a43 100644
--- a/src/heap/mark-compact.h
+++ b/src/heap/mark-compact.h
@@ -512,6 +512,10 @@ class MarkCompactCollector {
   static const uint32_t kSingleFreeEncoding = 0;
   static const uint32_t kMultiFreeEncoding = 1;
 
+  // If the number of deleted slots in a JSWeakCollection exceeds the number
+  // of entries / 2^(factor), we rehash the table.
+  static const int kJSWeakCollectionLoadFactorExp = 1;
+
   static inline bool IsMarked(Object* obj);
   static bool IsUnmarkedHeapObjectWithHeap(Heap* heap, Object** p);