shell.cc race condition

BUG=v8:3850
LOG=N

Review URL: https://codereview.chromium.org/892083002

Cr-Commit-Position: refs/heads/master@{#26375}

AUTHORS

@@ -52,6 +52,7 @@ Fedor Indutny <fedor@indutny.com>
 Felix Geisendörfer <haimuiba@gmail.com>
 Filipe David Manana <fdmanana@gmail.com>
 Geoffrey Garside <ggarside@gmail.com>
+Han Choongwoo <cwhan.tunz@gmail.com>
 Hirofumi Mako <mkhrfm@gmail.com>
 Ioseb Dzmanashvili <ioseb.dzmanashvili@gmail.com>
 Isiah Meadows <impinball@gmail.com>

samples/shell.cc

@@ -98,6 +98,7 @@ int main(int argc, char* argv[]) {
     result = RunMain(isolate, argc, argv);
     if (run_shell) RunShell(context);
   }
+  isolate->Dispose();
   v8::V8::Dispose();
   v8::V8::ShutdownPlatform();
   delete platform;