Add Core::ReplaceHandleWithReducedRights().

mojo/edk/system/message_pipe_dispatcher.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "mojo/edk/system/message_pipe_dispatcher.h"
 
 #include <utility>
 
 #include "base/logging.h"
 #include "mojo/edk/system/configuration.h"
@@ skipping 126 matching lines @@
   port_ = kInvalidPort;
 }
 
 RefPtr<Dispatcher>
 MessagePipeDispatcher::CreateEquivalentDispatcherAndCloseImplNoLock(
     MessagePipe* message_pipe,
     unsigned port) {
   mutex().AssertHeld();
 
   // "We" are being sent over our peer.
+  // If |message_pipe| is null, the |if| condition below should be false.
+  DCHECK(message_pipe_.get());
   if (message_pipe == message_pipe_.get()) {
     // A message pipe dispatcher can't be sent over itself (this should be
     // disallowed by |Core|). Note that |port| is the destination port.
     DCHECK_EQ(port, port_);
     // In this case, |message_pipe_|'s mutex should already be held!
     message_pipe_->CancelAllStateNoLock(port_);
   } else {
     CancelAllStateNoLock();
   }
 
@@ skipping 76 matching lines @@
 
   bool rv = message_pipe_->EndSerialize(port_, channel, destination,
                                         actual_size, platform_handles);
   message_pipe_ = nullptr;
   port_ = kInvalidPort;
   return rv;
 }
 
 }  // namespace system
 }  // namespace mojo