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

mojo/edk/system/waiter.cc

Index: mojo/edk/system/waiter.cc
diff --git a/mojo/edk/system/waiter.cc b/mojo/edk/system/waiter.cc
index e254b5ca2be5b2b1febca388811ce658e72cbaa1..3b67999a40c3155f9e425c361d71b290981570e8 100644
--- a/mojo/edk/system/waiter.cc
+++ b/mojo/edk/system/waiter.cc
@@ -92,13 +92,13 @@ MojoResult Waiter::Wait(MojoDeadline deadline,
   return MojoResultForAwakeReason(awake_reason_);
 }
 
-bool Waiter::Awake(uint64_t context,
+void Waiter::Awake(uint64_t context,
                    AwakeReason reason,
                    const HandleSignalsState& signals_state) {
   MutexLocker locker(&mutex_);
 
   if (awoken_)
-    return true;
+    return;
 
   awoken_ = true;
   awake_reason_ = reason;
@@ -107,7 +107,6 @@ bool Waiter::Awake(uint64_t context,
   cv_.Signal();
   // |cv_.Wait()|/|cv_.WaitWithTimeout()| will return after |mutex_| is
   // released.
-  return true;
 }
 
 }  // namespace system