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

base/run_loop.cc

Index: base/run_loop.cc
diff --git a/base/run_loop.cc b/base/run_loop.cc
index af2c568d467d38a36739efc9a86071e7ef32ba7d..4e425c9339b2c5662d8fc67dfabd01b73fca885f 100644
--- a/base/run_loop.cc
+++ b/base/run_loop.cc
@@ -68,6 +68,9 @@ bool RunLoop::BeforeRun() {
   run_depth_ = previous_run_loop_? previous_run_loop_->run_depth_ + 1 : 1;
   loop_->run_loop_ = this;
 
+  if (run_depth_ > 1)
+    loop_->NotifyBeginNestedLoop();
+
   running_ = true;
   return true;
 }