Mojo: Add a MessageInTransit::Swap() method and remove the "PassContents" ctor.

mojo/system/proxy_message_pipe_endpoint.cc

Index: mojo/system/proxy_message_pipe_endpoint.cc
diff --git a/mojo/system/proxy_message_pipe_endpoint.cc b/mojo/system/proxy_message_pipe_endpoint.cc
index ebe6bb44d929a5abdc7f58b93847174c0e182ad3..f3a3b34ba99c21582931cb810d5c3e1206f0dc7e 100644
--- a/mojo/system/proxy_message_pipe_endpoint.cc
+++ b/mojo/system/proxy_message_pipe_endpoint.cc
@@ -21,10 +21,8 @@ ProxyMessagePipeEndpoint::ProxyMessagePipeEndpoint()
 ProxyMessagePipeEndpoint::ProxyMessagePipeEndpoint(
     LocalMessagePipeEndpoint* local_message_pipe_endpoint,
     bool is_peer_open)
-    : is_running_(false),
-      is_peer_open_(is_peer_open),
-      paused_message_queue_(MessageInTransitQueue::PassContents(),
-                            local_message_pipe_endpoint->message_queue()) {
+    : is_running_(false), is_peer_open_(is_peer_open) {
+  paused_message_queue_.Swap(local_message_pipe_endpoint->message_queue());
   local_message_pipe_endpoint->Close();
 }