Makes InputMethodMus deal correctly with focus changes

ui/aura/mus/input_method_mus.cc

 // Copyright (c) 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/aura/mus/input_method_mus.h"
 
 #include <utility>
 
 #include "base/memory/ptr_util.h"
 #include "services/ui/public/interfaces/constants.mojom.h"
@@ skipping 177 matching lines @@
   for (auto& callback_ptr : pending_callbacks_) {
     if (callback_ptr)
       callback_ptr->Run(EventResult::UNHANDLED);
   }
   pending_callbacks_.clear();
 }
 
 void InputMethodMus::ProcessKeyEventCallback(
     const ui::KeyEvent& event,
     bool handled) {
+  // Remove the callback as DispatchKeyEventPostIME() may lead to calling
+  // AckPendingCallbacksUnhandled(), which mutates |pending_callbacks_|.
+  DCHECK(!pending_callbacks_.empty());
+  std::unique_ptr<EventResultCallback> ack_callback =
+      std::move(pending_callbacks_.front());
+  pending_callbacks_.pop_front();
   EventResult event_result;
   if (!handled) {
     // If not handled by IME, try dispatching the event to delegate to see if
     // any client-side post-ime processing needs to be done. This includes cases
     // like backspace, return key, etc.
     std::unique_ptr<ui::Event> event_clone = ui::Event::Clone(event);
     DispatchKeyEventPostIME(event_clone->AsKeyEvent());
     event_result =
         event_clone->handled() ? EventResult::HANDLED : EventResult::UNHANDLED;
   } else {
     event_result = EventResult::HANDLED;
   }
-  DCHECK(!pending_callbacks_.empty());
-  std::unique_ptr<EventResultCallback> ack_callback =
-      std::move(pending_callbacks_.front());
-  pending_callbacks_.pop_front();
   // |ack_callback| can be null if the standard form of DispatchKeyEvent() is
   // called instead of the version which provides a callback. In mus+ash we
   // use the version with callback, but some unittests use the standard form.
   if (ack_callback)
     ack_callback->Run(event_result);
 }
 
 }  // namespace aura