mus: Fix handled status in UI event ack, add MessageLoop::NestingObserver

base/message_loop/message_loop.h

Index: base/message_loop/message_loop.h
diff --git a/base/message_loop/message_loop.h b/base/message_loop/message_loop.h
index 802fae0dd191e64034c1779c81263a83d1fe6492..24d5b46aacac7bb5ea54b6dff894bddb9cb9058f 100644
--- a/base/message_loop/message_loop.h
+++ b/base/message_loop/message_loop.h
@@ -133,6 +133,7 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
   // Creates the default MessagePump based on |type|. Caller owns return
   // value.
   static std::unique_ptr<MessagePump> CreateMessagePumpForType(Type type);
+
   // A DestructionObserver is notified when the current MessageLoop is being
   // destroyed.  These observers are notified prior to MessageLoop::current()
   // being changed to return NULL.  This gives interested parties the chance to
@@ -157,6 +158,19 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
   // DestructionObserver is receiving a notification callback.
   void RemoveDestructionObserver(DestructionObserver* destruction_observer);
 
+  // A NestingObserver is notified when a nested message loop begins. The
+  // observers are notified before the first task is processed.
+  class BASE_EXPORT NestingObserver {
+   public:
+    virtual void OnBeginNestedMessageLoop() = 0;
+
+   protected:
+    virtual ~NestingObserver();
+  };
+
+  void AddNestingObserver(NestingObserver* observer);
+  void RemoveNestingObserver(NestingObserver* observer);
+
   // NOTE: Deprecated; prefer task_runner() and the TaskRunner interfaces.
   // TODO(skyostil): Remove these functions (crbug.com/465354).
   //
@@ -473,6 +487,9 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
   // If message_histogram_ is NULL, this is a no-op.
   void HistogramEvent(int event);
 
+  // Notify observers that a nested message loop is starting.
+  void NotifyBeginNestedLoop();
+
   // MessagePump::Delegate methods:
   bool DoWork() override;
   bool DoDelayedWork(TimeTicks* next_delayed_work_time) override;
@@ -507,6 +524,8 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
 
   ObserverList<DestructionObserver> destruction_observers_;
 
+  ObserverList<NestingObserver> nesting_observers_;
+
   // A recursion block that prevents accidentally running additional tasks when
   // insider a (accidentally induced?) nested message pump.
   bool nestable_tasks_allowed_;