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

mojo/system/message_in_transit_queue.h

Index: mojo/system/message_in_transit_queue.h
diff --git a/mojo/system/message_in_transit_queue.h b/mojo/system/message_in_transit_queue.h
index 2c1f4c70591b60a39780b930d0b241607a3f818d..a32f424bc93399c668e8ae8bbc6eac8da2a31c40 100644
--- a/mojo/system/message_in_transit_queue.h
+++ b/mojo/system/message_in_transit_queue.h
@@ -21,12 +21,6 @@ namespace system {
 class MOJO_SYSTEM_IMPL_EXPORT MessageInTransitQueue {
  public:
   MessageInTransitQueue();
-
-  struct PassContents {};
-  // Constructor that takes over the contents of another
-  // |MessageInTransitQueue|, leaving it empty.
-  MessageInTransitQueue(PassContents, MessageInTransitQueue* other);
-
   ~MessageInTransitQueue();
 
   bool IsEmpty() const { return queue_.empty(); }
@@ -50,6 +44,9 @@ class MOJO_SYSTEM_IMPL_EXPORT MessageInTransitQueue {
 
   void Clear();
 
+  // Efficiently swaps contents with |*other|.
+  void Swap(MessageInTransitQueue* other);
+
  private:
   // TODO(vtl): When C++11 is available, switch this to a deque of
   // |scoped_ptr|/|unique_ptr|s.