Enable pepper threading by default.

ppapi/proxy/plugin_dispatcher.cc

Index: ppapi/proxy/plugin_dispatcher.cc
diff --git a/ppapi/proxy/plugin_dispatcher.cc b/ppapi/proxy/plugin_dispatcher.cc
index 1c1eedcc5d6dce66b90bce4392284a19b7c8f78b..bf3d7930cc38744f5956859fd80823f85731f9f8 100644
--- a/ppapi/proxy/plugin_dispatcher.cc
+++ b/ppapi/proxy/plugin_dispatcher.cc
@@ -274,6 +274,10 @@ thunk::ResourceCreationAPI* PluginDispatcher::GetResourceCreationAPI() {
 void PluginDispatcher::DispatchResourceReply(
     const ppapi::proxy::ResourceMessageReplyParams& reply_params,
     const IPC::Message& nested_msg) {
+  // We need to grab the proxy lock to ensure that we don't collide with the
+  // plugin making pepper calls on a different thread.
+  ProxyAutoLock lock;
+
   Resource* resource = PpapiGlobals::Get()->GetResourceTracker()->GetResource(
       reply_params.pp_resource());
   if (!resource) {