Revert 187340 "PPAPI: Remove threading options; it's always on"

ppapi/proxy/ppp_instance_private_proxy_unittest.cc

Index: ppapi/proxy/ppp_instance_private_proxy_unittest.cc
===================================================================
--- ppapi/proxy/ppp_instance_private_proxy_unittest.cc	(revision 187345)
+++ ppapi/proxy/ppp_instance_private_proxy_unittest.cc	(working copy)
@@ -147,7 +147,15 @@
 
 }  // namespace
 
+// TODO(raymes): This #ifdef is only here because we check the state of the
+// plugin globals on the main thread, rather than the plugin thread which causes
+// the thread checker to fail. Once ENABLE_PEPPER_THREADING is the default,
+// this will be safe to do anyway, so we can remove this.
+#ifdef ENABLE_PEPPER_THREADING
 TEST_F(PPP_Instance_Private_ProxyTest, PPPInstancePrivate) {
+#else
+TEST_F(PPP_Instance_Private_ProxyTest, DISABLED_PPPInstancePrivate) {
+#endif
   // This test controls its own instance; we can't use the one that
   // PluginProxyTestHarness provides.
   ASSERT_NE(kInstance, pp_instance());