Have panels respond to changes in work area on Linux.

ui/base/x/work_area_watcher_x.h

Index: ui/base/x/work_area_watcher_x.h
diff --git a/ui/base/x/work_area_watcher_x.h b/ui/base/x/work_area_watcher_x.h
new file mode 100644
index 0000000000000000000000000000000000000000..428f851ea6cd153cd62633e33f90add3809820f6
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+++ b/ui/base/x/work_area_watcher_x.h
@@ -0,0 +1,53 @@
+// Copyright (c) 2011 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_BASE_X_WORK_AREA_WATCHER_X_H_
+#define UI_BASE_X_WORK_AREA_WATCHER_X_H_
+#pragma once
+
+#include "base/memory/singleton.h"
+#include "base/observer_list.h"
+#include "ui/base/ui_export.h"
+#include "ui/base/x/x11_util.h"
+
+namespace ui {
+
+// This is a helper class that is used to keep track of changes to work area.
+// Add an observer to track changes.
+class UI_EXPORT WorkAreaWatcherX {
+ public:
+  class Observer {
+   public:
+    virtual void WorkAreaChanged() = 0;
+
+   protected:
+    virtual ~Observer();
+  };
+
+  static WorkAreaWatcherX* GetInstance();
+  static void AddObserver(Observer* observer);
+  static void RemoveObserver(Observer* observer);
+
+  // Gets the atom for the default display for the propert this class is
+  // watching for.
+  static Atom GetPropertyAtom();
+
+  // Notify observers that the work area has changed.
+  static void Notify();
+
+ private:
+  friend struct DefaultSingletonTraits<WorkAreaWatcherX>;
+
+  WorkAreaWatcherX();
+  ~WorkAreaWatcherX();
+  void NotifyWorkAreaChanged();
+
+  ObserverList<Observer> observers_;
+
+  DISALLOW_COPY_AND_ASSIGN(WorkAreaWatcherX);
+};
+
+}  // namespace ui
+
+#endif  // UI_BASE_X_WORK_AREA_WATCHER_X_H_