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

R=darin@chromium.org

Review URL: https://codereview.chromium.org/585633004

Cr-Commit-Position: refs/heads/master@{#295730}

mojo/system/message_in_transit_queue.cc

@@ -13,11 +13,6 @@ namespace system {
 MessageInTransitQueue::MessageInTransitQueue() {
 }
 
-MessageInTransitQueue::MessageInTransitQueue(PassContents,
-                                             MessageInTransitQueue* other) {
-  queue_.swap(other->queue_);
-}
-
 MessageInTransitQueue::~MessageInTransitQueue() {
   if (!IsEmpty()) {
     LOG(WARNING) << "Destroying nonempty message queue";
@@ -29,5 +24,9 @@ void MessageInTransitQueue::Clear() {
   STLDeleteElements(&queue_);
 }
 
+void MessageInTransitQueue::Swap(MessageInTransitQueue* other) {
+  queue_.swap(other->queue_);
+}
+
 }  // namespace system
 }  // namespace mojo

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.

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();
 }