Fix memory leaks in ~Zone and ~Isolate

src/isolate.cc

Index: src/isolate.cc
diff --git a/src/isolate.cc b/src/isolate.cc
index 9ca61177ba7b41914987dbf0aaba503b50ab2773..eae812bcd94bdc35c4d48a497a17375fc332fd5f 100644
--- a/src/isolate.cc
+++ b/src/isolate.cc
@@ -1533,6 +1533,9 @@ void Isolate::SetIsolateThreadLocals(Isolate* isolate,
 Isolate::~Isolate() {
   TRACE_ISOLATE(destructor);
 
+  // Has to be called while counters_ are still alive.
+  zone_.DeleteKeptSegment();
+
   delete unicode_cache_;
   unicode_cache_ = NULL;
 
@@ -1591,6 +1594,9 @@ Isolate::~Isolate() {
   delete global_handles_;
   global_handles_ = NULL;
 
+  delete external_reference_table_;
+  external_reference_table_ = NULL;
+
 #ifdef ENABLE_DEBUGGER_SUPPORT
   delete debugger_;
   debugger_ = NULL;