Plug leaked string stream debug object cache in Isolate.

src/isolate.cc

Index: src/isolate.cc
diff --git a/src/isolate.cc b/src/isolate.cc
index e67eb0c62d1e4eee8eb2c98efcb8892b56c2853c..8dfebaa3cd1d11bc7770458773d44e8c4c117124 100644
--- a/src/isolate.cc
+++ b/src/isolate.cc
@@ -29,6 +29,7 @@
 
 #include "v8.h"
 
+#include "allocation-inl.h"
 #include "ast.h"
 #include "bootstrapper.h"
 #include "codegen.h"
@@ -2002,6 +2003,9 @@ Isolate::~Isolate() {
   delete global_handles_;
   global_handles_ = NULL;
 
+  delete string_stream_debug_object_cache_;
+  string_stream_debug_object_cache_ = NULL;
+
   delete external_reference_table_;
   external_reference_table_ = NULL;