Fix race condition in test-thread-termination.
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: b158db1e4b4ab85d4c9e510fdef4b1e8c614b15b@11217 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh