Declare virtual destructor in Nacl plugin CallbackSource to fix mac build

ppapi/native_client/src/trusted/plugin/pnacl_coordinator.cc

Index: ppapi/native_client/src/trusted/plugin/pnacl_coordinator.cc
diff --git a/ppapi/native_client/src/trusted/plugin/pnacl_coordinator.cc b/ppapi/native_client/src/trusted/plugin/pnacl_coordinator.cc
index 7ef290263217bf00d74327ce8d7f72f5fb53090a..2ce0f29ed5b4da054f2e0ccd4c9c05854cb04fa1 100644
--- a/ppapi/native_client/src/trusted/plugin/pnacl_coordinator.cc
+++ b/ppapi/native_client/src/trusted/plugin/pnacl_coordinator.cc
@@ -166,6 +166,12 @@ class PnaclLDManifest : public Manifest {
 //////////////////////////////////////////////////////////////////////
 //  The coordinator class.
 //////////////////////////////////////////////////////////////////////
+
+// Out-of-line destructor to keep it from getting put in every .o where
+// callback_source.h is included
+template<>
+CallbackSource<FileStreamData>::~CallbackSource() {}
+
 PnaclCoordinator* PnaclCoordinator::BitcodeToNative(
     Plugin* plugin,
     const nacl::string& pexe_url,