Add different behavior to AwakableList for "persistent" vs "one-shot" awakables.

mojo/edk/system/awakable.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "mojo/edk/system/awakable.h"
 
 #include "base/logging.h"
 
 namespace mojo {
 namespace system {
 
 // static
 MojoResult Awakable::MojoResultForAwakeReason(AwakeReason reason) {
   switch (reason) {
     case AwakeReason::SATISFIED:
       return MOJO_RESULT_OK;
     case AwakeReason::UNSATISFIABLE:
       return MOJO_RESULT_FAILED_PRECONDITION;
     case AwakeReason::CANCELLED:
       return MOJO_RESULT_CANCELLED;
+    case AwakeReason::CHANGED:
+      break;
   }
   NOTREACHED();
   return MOJO_RESULT_INTERNAL;
 }
 
 }  // namespace system
 }  // namespace mojo