ipc: Add the inner class OutputElement to Channel.

ipc/ipc_channel.h

Index: ipc/ipc_channel.h
diff --git a/ipc/ipc_channel.h b/ipc/ipc_channel.h
index c74d084baac5a23099037e49594acbd594862c1b..552f2c07e982123b033e6b43f27df3e05c9694b7 100644
--- a/ipc/ipc_channel.h
+++ b/ipc/ipc_channel.h
@@ -240,6 +240,27 @@ class IPC_EXPORT Channel : public Endpoint {
   // process such that it acts similar to if it was exec'd, for tests.
   static void NotifyProcessForkedForTesting();
 #endif
+
+ protected:
+  // An OutputElement is a wrapper around a Message or raw buffer while it is
+  // waiting to be passed to the system's underlying IPC mechanism.
+  class OutputElement {
+   public:
+    // Takes ownership of message.
+    OutputElement(Message* message);
+    // Takes ownership of the buffer. |buffer| is freed via free(), so it
+    // must be malloced.
+    OutputElement(void* buffer, size_t length);
+    ~OutputElement();
+    size_t size() const { return message_ ? message_->size() : length_; }
+    const void* data() const { return message_ ? message_->data() : buffer_; }
+    const Message* get_message() const { return message_.get(); }
+
+   private:
+    scoped_ptr<const Message> message_;
+    void* buffer_;
+    size_t length_;
+  };
 };
 
 #if defined(OS_POSIX)