Changes how window is updated when display changes in mash

ui/display/display_list.h

 // Copyright 2016 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_DISPLAY_DISPLAY_LIST_H_
 #define UI_DISPLAY_DISPLAY_LIST_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 46 matching lines @@
 
   Displays::const_iterator FindDisplayById(int64_t id) const;
 
   Displays::const_iterator GetPrimaryDisplayIterator() const;
 
   // Internally increments a counter that while non-zero results in observers
   // not being called for any changes to the displays. It is assumed once
   // callers release the last lock they call the observers appropriately.
   std::unique_ptr<DisplayListObserverLock> SuspendObserverUpdates();
 
-  // Updates the cached display based on display.id().
-  void UpdateDisplay(const Display& display);
+  // Updates the cached display based on display.id(). This returns a bitmask
+  // of the changed values suitable for passing to
+  // DisplayObserver::OnDisplayMetricsChanged().
+  uint32_t UpdateDisplay(const Display& display);
 
   // Updates the cached display based on display.id(). Also updates the primary
-  // display if |type| indicates |display| is the primary display.
-  void UpdateDisplay(const Display& display, Type type);
+  // display if |type| indicates |display| is the primary display. See single
+  // argument version for description of return value.
+  uint32_t UpdateDisplay(const Display& display, Type type);
 
   // Adds a new Display.
   void AddDisplay(const Display& display, Type type);
 
   // Removes the Display with the specified id.
   void RemoveDisplay(int64_t id);
 
   base::ObserverList<DisplayObserver>* observers() { return &observers_; }
 
  private:
@@ skipping 14 matching lines @@
   base::ObserverList<DisplayObserver> observers_;
 
   int observer_suspend_lock_count_ = 0;
 
   DISALLOW_COPY_AND_ASSIGN(DisplayList);
 };
 
 }  // namespace display
 
 #endif  // UI_DISPLAY_DISPLAY_LIST_H_