Only wait for previous frame's motion vector if needed.

Change-Id: Iecce685a33b64844446c0009f21bc85566d7469f
diff --git a/vp9/decoder/vp9_dthread.c b/vp9/decoder/vp9_dthread.c
index d2a2b81..0904e93 100644
--- a/vp9/decoder/vp9_dthread.c
+++ b/vp9/decoder/vp9_dthread.c
@@ -38,9 +38,15 @@
 void vp9_frameworker_signal_stats(VP9Worker *const worker) {
 #if CONFIG_MULTITHREAD
   FrameWorkerData *const worker_data = worker->data1;
-  // TODO(hkuang): Investigate using broadcast or signal.
+
+// TODO(hkuang): Fix the pthread_cond_broadcast in windows wrapper.
+#if defined(_WIN32) && !HAVE_PTHREAD_H
   pthread_cond_signal(&worker_data->stats_cond);
 #else
+  pthread_cond_broadcast(&worker_data->stats_cond);
+#endif
+
+#else
   (void)worker;
 #endif
 }