Remove unnecessary SendContext type

Also, add a few more comments.

Change-Id: Iaefb54706c6b218688d0ebfaac3c1484799b3a87
diff --git a/aos/events/raw-event-loop.h b/aos/events/raw-event-loop.h
index fd9ea8f..be605e9 100644
--- a/aos/events/raw-event-loop.h
+++ b/aos/events/raw-event-loop.h
@@ -47,21 +47,19 @@
 // Send() or Free().
 class RawSender {
  public:
-  class SendContext;
-
   RawSender() {}
   virtual ~RawSender() {}
 
-  virtual SendContext *GetContext() = 0;
+  virtual aos::Message *GetMessage() = 0;
 
-  virtual void Free(SendContext *context) = 0;
+  virtual void Free(aos::Message *msg) = 0;
 
-  virtual bool Send(SendContext *context) = 0;
+  virtual bool Send(aos::Message *msg) = 0;
 
   // Call operator that calls Free().
   template <typename T>
   void operator()(T *t) {
-    Free(reinterpret_cast<SendContext *>(t));
+    Free(t);
   }
 
   virtual const char *name() const = 0;