mash: make Env::last_mouse_location() accurate for Mus

ui/aura/mus/mus_mouse_location_updater.h

Index: ui/aura/mus/mus_mouse_location_updater.h
diff --git a/ui/aura/mus/mus_mouse_location_updater.h b/ui/aura/mus/mus_mouse_location_updater.h
new file mode 100644
index 0000000000000000000000000000000000000000..b05772273d7e7dbd9e7b3b236a4b107829b7ce05
--- /dev/null
+++ b/ui/aura/mus/mus_mouse_location_updater.h
@@ -0,0 +1,48 @@
+// Copyright 2017 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.
+
+#ifndef UI_AURA_MUS_MUS_MOUSE_LOCATION_UPDATER_H_
+#define UI_AURA_MUS_MUS_MOUSE_LOCATION_UPDATER_H_
+
+#include "base/macros.h"
+#include "base/message_loop/message_loop.h"
+
+namespace ui {
+class Event;
+}
+
+namespace aura {
+
+// MusMouseLocationUpdater is responsible for updating
+// Env::last_mouse_location(), as well as determining when
+// Env::last_mouse_location() should use
+// WindowTreeClient::GetCursorScreenPoint(). While processing an event
+// Env uses the value from the current event, otherwise Env uses
+// WindowTreeClient::GetCursorScreenPoint(). If a nested message loop is
+// started while processing an event Env uses GetCursorScreenPoint().
+class MusMouseLocationUpdater : public base::MessageLoop::NestingObserver {
+ public:
+  MusMouseLocationUpdater();
+  ~MusMouseLocationUpdater() override;
+
+  // Called from WindowEventDispatcher on starting/stopping processing of
+  // events.
+  void OnEventProcessingStarted(const ui::Event& event);
+  void OnEventProcessingFinished();
+
+ private:
+  void StopObserving();
+
+  // base::MessageLoop::NestingObserver:
+  void OnBeginNestedMessageLoop() override;
+
+  // Set to true once a valid mouse event is encountered.
+  bool observing_message_loop_ = false;
+
+  DISALLOW_COPY_AND_ASSIGN(MusMouseLocationUpdater);
+};
+
+}  // namespace aura
+
+#endif  // UI_AURA_MUS_MUS_MOUSE_LOCATION_UPDATER_H_