Fix race condition in test-thread-termination.

The test case failed to stop the preemption thread before destroying the
isolate which led to a race condition where the isolate was accessed
after it had been destroyed.

R=ulan@chromium.org
BUG=v8:2049
TEST=cctest/test-thread-termination/TerminateMultipleV8ThreadsDefaultIsolate

Review URL: https://chromiumcodereview.appspot.com/9969092

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11217 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

test/cctest/test-thread-termination.cc

@@ -255,6 +255,10 @@ TEST(TerminateMultipleV8ThreadsDefaultIsolate) {
     threads[i]->Join();
     delete threads[i];
   }
+  {
+    v8::Locker locker;
+    v8::Locker::StopPreemption();
+  }
 
   delete semaphore;
   semaphore = NULL;