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

ipc/attachment_broker_win.cc

Index: ipc/attachment_broker_win.cc
diff --git a/ipc/attachment_broker_win.cc b/ipc/attachment_broker_win.cc
index a4cb067afa69c170afea18bdd1034ac17f551d63..1bc013dd03048a334838ba2f0f9271d12e7c95b8 100644
--- a/ipc/attachment_broker_win.cc
+++ b/ipc/attachment_broker_win.cc
@@ -27,10 +27,7 @@ bool AttachmentBrokerWin::SendAttachmentToProcess(
           static_cast<const internal::HandleAttachmentWin*>(attachment);
       internal::HandleAttachmentWin::WireFormat format =
           handle_attachment->GetWireFormat(destination_process);
-      // TODO(erikchen): Replace the call to base::Process::Current().Pid() with
-      // a non-forgeable mechanism. http://crbug.com/513431.
-      return sender_->Send(new AttachmentBrokerMsg_DuplicateWinHandle(
-          format, base::Process::Current().Pid()));
+      return sender_->Send(new AttachmentBrokerMsg_DuplicateWinHandle(format));
   }
   return false;
 }