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

mojo/edk/system/async_waiter.h

Index: mojo/edk/system/async_waiter.h
diff --git a/mojo/edk/system/async_waiter.h b/mojo/edk/system/async_waiter.h
index 5e405bf790503b249dd8d83ee734f43770fc022f..fd8e2bc9fdfa4b86a72b55e4e80a6485c5f52ca4 100644
--- a/mojo/edk/system/async_waiter.h
+++ b/mojo/edk/system/async_waiter.h
@@ -14,7 +14,9 @@
 namespace mojo {
 namespace system {
 
-// An |Awakable| implementation that just calls a given callback object.
+// An |Awakable| implementation that just calls a given callback object. It
+// should be used in a non-persistent way (i.e., |Awake()| should be called at
+// most once by each source, and only for "leading edges").
 class AsyncWaiter final : public Awakable {
  public:
   using AwakeCallback = std::function<void(MojoResult)>;
@@ -25,7 +27,7 @@ class AsyncWaiter final : public Awakable {
 
  private:
   // |Awakable| implementation:
-  bool Awake(uint64_t context,
+  void Awake(uint64_t context,
              AwakeReason reason,
              const HandleSignalsState& signals_state) override;