Make IPC channels track their peer process ID

ipc/ipc_channel_posix.cc

Index: ipc/ipc_channel_posix.cc
===================================================================
--- ipc/ipc_channel_posix.cc	(revision 130443)
+++ ipc/ipc_channel_posix.cc	(working copy)
@@ -303,6 +303,7 @@
                                   Mode mode, Listener* listener)
     : ChannelReader(listener),
       mode_(mode),
+      peer_pid_(base::kNullProcessId),
       is_blocked_on_write_(false),
       waiting_connect_(true),
       message_send_bytes_written_(0),
@@ -1081,6 +1082,7 @@
     CHECK(descriptor.auto_close);
   }
 #endif  // IPC_USES_READWRITE
+  peer_pid_ = pid;
   listener()->OnChannelConnected(pid);
 }
 
@@ -1128,6 +1130,10 @@
   channel_impl_->set_listener(listener);
 }
 
+base::ProcessId Channel::peer_pid() const {
+  return channel_impl_->peer_pid();
+}
+
 bool Channel::Send(Message* message) {
   return channel_impl_->Send(message);
 }