Add Core::ReplaceHandleWithReducedRights().

mojo/edk/system/dispatcher.h

Index: mojo/edk/system/dispatcher.h
diff --git a/mojo/edk/system/dispatcher.h b/mojo/edk/system/dispatcher.h
index 30e2d5d8934e37a01d41a92e4634285042eaaf41..231e77063608ea66298ff5693238f8f6793fa268 100644
--- a/mojo/edk/system/dispatcher.h
+++ b/mojo/edk/system/dispatcher.h
@@ -281,6 +281,11 @@ class Dispatcher : public util::RefCountedThreadSafe<Dispatcher> {
   // |CancelAllStateNoLock()| (or equivalent); unlike |CloseNoLock()|,
   // |CreateEquivalentDispatcherAndCloseNoLock()| does not do this
   // automatically.
+  //
+  // If the new dispatcher will be enqueued on a message pipe, then
+  // |message_pipe|/|port| will be provided (since there may be some
+  // special-casing necessary due to locking considerations); otherwise,
+  // |message_pipe| will be null (and |port| will be meaningless).
   virtual util::RefPtr<Dispatcher> CreateEquivalentDispatcherAndCloseImplNoLock(
       MessagePipe* message_pipe,
       unsigned port) MOJO_EXCLUSIVE_LOCKS_REQUIRED(mutex_) = 0;