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

ipc/ipc_channel_posix.cc

Index: ipc/ipc_channel_posix.cc
diff --git a/ipc/ipc_channel_posix.cc b/ipc/ipc_channel_posix.cc
index b28cc527d62028c56d2b383c88ac2da2f356dfee..1dd5702a229db75110c4ce97e0959f24c866ec29 100644
--- a/ipc/ipc_channel_posix.cc
+++ b/ipc/ipc_channel_posix.cc
@@ -951,6 +951,10 @@ void ChannelPosix::HandleInternalMessage(const Message& msg) {
   }
 }
 
+base::ProcessId ChannelPosix::GetSenderPID() {
+  return GetPeerPID();
+}
+
 void ChannelPosix::Close() {
   // Close can be called multiple time, so we need to make sure we're
   // idempotent.