Remove dead code guarded by OS(WINDOWS) && USE(PTHREADS).

Blink does not support using Pthreads on Windows.

Review URL: https://chromiumcodereview.appspot.com/22433006

git-svn-id: svn://svn.chromium.org/blink/trunk@155901 bbb929c8-8fbe-4397-9dbb-9b2b20218538