Let IPC::SyncMessageFilter take advantage of thread-safe Send.

ppapi/nacl_irt/manifest_service.cc

Index: ppapi/nacl_irt/manifest_service.cc
diff --git a/ppapi/nacl_irt/manifest_service.cc b/ppapi/nacl_irt/manifest_service.cc
index 8a6ec1f641835bae0181f12acb432585a3a8ad62..41e8601f3b5af96c5bee72ba73d5e5ded2cd6ecb 100644
--- a/ppapi/nacl_irt/manifest_service.cc
+++ b/ppapi/nacl_irt/manifest_service.cc
@@ -28,7 +28,8 @@ namespace ppapi {
 class ManifestMessageFilter : public IPC::SyncMessageFilter {
  public:
   ManifestMessageFilter(base::WaitableEvent* shutdown_event)
-      : SyncMessageFilter(shutdown_event),
+      : SyncMessageFilter(shutdown_event,
+                          false /* is_channel_send_thread_safe */),
         connected_event_(
             true /* manual_reset */, false /* initially_signaled */) {
   }