Keyboard Lock Host implementation

components/keyboard_lock/active_key_event_filter_tracker.h

Index: components/keyboard_lock/active_key_event_filter_tracker.h
diff --git a/components/keyboard_lock/active_key_event_filter_tracker.h b/components/keyboard_lock/active_key_event_filter_tracker.h
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+++ b/components/keyboard_lock/active_key_event_filter_tracker.h
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+// 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 COMPONENTS_KEYBOARD_LOCK_ACTIVE_KEY_EVENT_FILTER_TRACKER_H_
+#define COMPONENTS_KEYBOARD_LOCK_ACTIVE_KEY_EVENT_FILTER_TRACKER_H_
+
+#include "base/threading/thread_checker.h"
+
+namespace keyboard_lock {
+
+class KeyEventFilter;
+
+// Tracks the active KeyEventFilter.
+// All public functions must be executed in one thread.
+class ActiveKeyEventFilterTracker final {
+ public:
+  ActiveKeyEventFilterTracker();
+  ~ActiveKeyEventFilterTracker();
+
+  void set(KeyEventFilter* filter);
+  // If |filter| == |filter_|, set |filter_| to nullptr. It ensures no matter
+  // whether LostFocus or GotFocus is fired first, we can always get the latest
+  // active filter.
+  void erase(const KeyEventFilter* filter);
+  KeyEventFilter* get() const;
+
+ private:
+  KeyEventFilter* filter_ = nullptr;
+  base::ThreadChecker thread_checker_;
+};
+
+}  // namespace keyboard_lock
+
+#endif  // COMPONENTS_KEYBOARD_LOCK_ACTIVE_KEY_EVENT_FILTER_TRACKER_H_