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

base/message_loop/message_loop.cc

Index: base/message_loop/message_loop.cc
diff --git a/base/message_loop/message_loop.cc b/base/message_loop/message_loop.cc
index 01e7512b19f09ce6d611c6b2db18161e55437dbf..f189637b2223b383c34bb0e4f17453dc9249aff4 100644
--- a/base/message_loop/message_loop.cc
+++ b/base/message_loop/message_loop.cc
@@ -121,6 +121,8 @@ MessageLoop::TaskObserver::~TaskObserver() {
 MessageLoop::DestructionObserver::~DestructionObserver() {
 }
 
+MessageLoop::NestingObserver::~NestingObserver() {}
+
 //------------------------------------------------------------------------------
 
 MessageLoop::MessageLoop(Type type)
@@ -263,6 +265,16 @@ void MessageLoop::RemoveDestructionObserver(
   destruction_observers_.RemoveObserver(destruction_observer);
 }
 
+void MessageLoop::AddNestingObserver(NestingObserver* observer) {
+  DCHECK_EQ(this, current());
+  nesting_observers_.AddObserver(observer);
+}
+
+void MessageLoop::RemoveNestingObserver(NestingObserver* observer) {
+  DCHECK_EQ(this, current());
+  nesting_observers_.RemoveObserver(observer);
+}
+
 void MessageLoop::PostTask(
     const tracked_objects::Location& from_here,
     const Closure& task) {
@@ -576,6 +588,11 @@ void MessageLoop::HistogramEvent(int event) {
 #endif
 }
 
+void MessageLoop::NotifyBeginNestedLoop() {
+  FOR_EACH_OBSERVER(NestingObserver, nesting_observers_,
+                    OnBeginNestedMessageLoop());
+}
+
 bool MessageLoop::DoWork() {
   if (!nestable_tasks_allowed_) {
     // Task can't be executed right now.