Correctly keep track of isolates for microtask execution

BUG=487155
R=haraken@chromium.org

Review URL: https://codereview.chromium.org/1161823002

git-svn-id: svn://svn.chromium.org/blink/trunk@195985 bbb929c8-8fbe-4397-9dbb-9b2b20218538
void WorkerThread::destroyIsolate()
{
    ASSERT(isCurrentThread());
    V8PerIsolateData::destroy(m_isolate);
    m_isolate = nullptr;
}

void WorkerThread::destroyIsolate()
{
    ASSERT(isCurrentThread());
    V8PerIsolateData::destroy(m_isolate);
    m_isolate = nullptr;
}
