[d8] Fix flakiness when calling quit() with isolates

Don't use exit(), use Shell::Exit() (which calls _exit() instead). This won't
run C++ static destructors, atexit() functions, etc., which can occasionally
cause flaky failures.

BUG=v8:4279
R=machenbach@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1294913005

Cr-Commit-Position: refs/heads/master@{#30229}

src/d8.cc

@@ -818,7 +818,7 @@ void Shell::QuitOnce(v8::FunctionCallbackInfo<v8::Value>* args) {
   CleanupWorkers();
 #endif  // !V8_SHARED
   OnExit(args->GetIsolate());
-  exit(exit_code);
+  Exit(exit_code);
 }