ipc: Create an unforgeable mechanism to get the process id of the sender.

ipc/ipc_channel_posix.h

 // Copyright (c) 2012 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.
 
 #ifndef IPC_IPC_CHANNEL_POSIX_H_
 #define IPC_IPC_CHANNEL_POSIX_H_
 
 #include "ipc/ipc_channel.h"
 
 #include <sys/socket.h>  // for CMSG macros
@@ skipping 65 matching lines @@
   void QueueHelloMessage();
   void CloseFileDescriptors(Message* msg);
   void QueueCloseFDMessage(int fd, int hops);
 
   // ChannelReader implementation.
   ReadState ReadData(char* buffer, int buffer_len, int* bytes_read) override;
   bool ShouldDispatchInputMessage(Message* msg) override;
   bool GetNonBrokeredAttachments(Message* msg) override;
   bool DidEmptyInputBuffers() override;
   void HandleInternalMessage(const Message& msg) override;
+  base::ProcessId GetSenderPID() override;
 
   // Finds the set of file descriptors in the given message.  On success,
   // appends the descriptors to the input_fds_ member and returns true
   //
   // Returns false if the message was truncated. In this case, any handles that
   // were sent will be closed.
   bool ExtractFileDescriptorsFromMsghdr(msghdr* msg);
 
   // Closes all handles in the input_fds_ list and clears the list. This is
   // used to clean up handles in error conditions to avoid leaking the handles.
@@ skipping 86 matching lines @@
 
   // |broker_| must outlive this instance.
   AttachmentBroker* broker_;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(ChannelPosix);
 };
 
 }  // namespace IPC
 
 #endif  // IPC_IPC_CHANNEL_POSIX_H_