Serializer: cache hashmaps on the isolate.

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index 1b0ab092f04da91374cb07f7d8b46da18aa75e49..074690ade8c32b33cc1aa14d76c18b65280df054 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -802,6 +802,7 @@ void Heap::CollectAllAvailableGarbage(const char* gc_reason) {
     DisallowHeapAllocation no_recursive_gc;
     isolate()->optimizing_compiler_thread()->Flush();
   }
+  isolate()->ClearSerializerData();
   mark_compact_collector()->SetFlags(kMakeHeapIterableMask |
                                      kReduceMemoryFootprintMask);
   isolate_->compilation_cache()->Clear();