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

ipc/ipc_message.h

Index: ipc/ipc_message.h
diff --git a/ipc/ipc_message.h b/ipc/ipc_message.h
index 878784718dd5c969ce0cebfb3f69bac6b4a18d06..73a5165aed219c70cadb7428f19fb1315db7e883 100644
--- a/ipc/ipc_message.h
+++ b/ipc/ipc_message.h
@@ -184,6 +184,9 @@ class IPC_EXPORT Message : public base::Pickle {
   // Whether the message has any brokerable attachments.
   bool HasBrokerableAttachments() const;
 
+  void set_sender_pid(base::ProcessId id) { sender_pid_ = id; }
+  base::ProcessId get_sender_pid() const { return sender_pid_; }
+
 #ifdef IPC_MESSAGE_LOG_ENABLED
   // Adds the outgoing time from Time::Now() at the end of the message and sets
   // a bit to indicate that it's been added.
@@ -263,6 +266,10 @@ class IPC_EXPORT Message : public base::Pickle {
     return attachment_set_.get();
   }
 
+  // The process id of the sender of the message. This member is populated with
+  // a valid value for every message dispatched to listeners.
+  base::ProcessId sender_pid_;
+
 #ifdef IPC_MESSAGE_LOG_ENABLED
   // Used for logging.
   mutable int64 received_time_;