Add more out of line copy ctors for complex classes.

mojo/edk/system/request_context.cc

Index: mojo/edk/system/request_context.cc
diff --git a/mojo/edk/system/request_context.cc b/mojo/edk/system/request_context.cc
index f4f70b5ec1fff6325884d377ca907f90ff5a159b..e3666b2269e8140f2d828f45cabbf9b7a128db76 100644
--- a/mojo/edk/system/request_context.cc
+++ b/mojo/edk/system/request_context.cc
@@ -90,6 +90,9 @@ RequestContext::WatchNotifyFinalizer::WatchNotifyFinalizer(
     : watcher(watcher), result(result), state(state) {
 }
 
+RequestContext::WatchNotifyFinalizer::WatchNotifyFinalizer(
+    const WatchNotifyFinalizer& other) = default;
+
 RequestContext::WatchNotifyFinalizer::~WatchNotifyFinalizer() {}
 
 }  // namespace edk