Mojo: Refactor some message pipe stuff.

mojo/system/message_pipe_endpoint.h

Index: mojo/system/message_pipe_endpoint.h
diff --git a/mojo/system/message_pipe_endpoint.h b/mojo/system/message_pipe_endpoint.h
index 835b96670789781625913dc1ad3cb3e0e776927c..4825b37160c2c686bdd440e21fe0416b3caa1998 100644
--- a/mojo/system/message_pipe_endpoint.h
+++ b/mojo/system/message_pipe_endpoint.h
@@ -37,20 +37,11 @@ class MOJO_SYSTEM_IMPL_EXPORT MessagePipeEndpoint {
   // All implementations must implement these.
   virtual void Close() = 0;
   virtual void OnPeerClose() = 0;
-  // Checks if |EnqueueMessage()| will be able to enqueue the given message
-  // (with the given set of dispatchers). |dispatchers| should be non-null only
-  // if it's nonempty. Returns |MOJO_RESULT_OK| if it will and an appropriate
-  // error code if it won't.
-  virtual MojoResult CanEnqueueMessage(
-      const MessageInTransit* message,
-      const std::vector<Dispatcher*>* dispatchers) = 0;
-  // Takes ownership of |message| and the contents of |dispatchers| (leaving
-  // it empty). This should only be called after |CanEnqueueMessage()| has
-  // indicated success. (Unlike |CanEnqueueMessage()|, |dispatchers| may be
-  // non-null but empty.)
-  virtual void EnqueueMessage(
+  // Implements |MessagePipe::EnqueueMessage()| (see its description for
+  // details).
+  virtual MojoResult EnqueueMessage(
       MessageInTransit* message,
-      std::vector<scoped_refptr<Dispatcher> >* dispatchers) = 0;
+      const std::vector<Dispatcher*>* dispatchers) = 0;
 
   // Implementations must override these if they represent a local endpoint,
   // i.e., one for which there's a |MessagePipeDispatcher| (and thus a handle).