release callback table memory in ~Isolate

R=svenpanne@chromium.org
BUG=

Review URL: https://codereview.chromium.org/16304008

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15075 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/isolate.cc

@@ -2007,6 +2007,9 @@ Isolate::~Isolate() {
   delete external_reference_table_;
   external_reference_table_ = NULL;
 
+  delete callback_table_;
+  callback_table_ = NULL;
+
 #ifdef ENABLE_DEBUGGER_SUPPORT
   delete debugger_;
   debugger_ = NULL;