Adds ability to suspend notifying observers when display list is changed

ui/display/display_list.h

Index: ui/display/display_list.h
diff --git a/ui/display/display_list.h b/ui/display/display_list.h
index c4b96ece813600a00358a2ee82945de0c14a576b..df748457c63eb85f196094e1f8a9915682d1874e 100644
--- a/ui/display/display_list.h
+++ b/ui/display/display_list.h
@@ -7,6 +7,7 @@
 
 #include <stdint.h>
 
+#include <memory>
 #include <vector>
 
 #include "base/observer_list.h"
@@ -16,8 +17,24 @@
 namespace display {
 
 class Display;
+class DisplayList;
 class DisplayObserver;
 
+// See description in DisplayLock::SuspendObserverUpdates.
+class DISPLAY_EXPORT DisplayListObserverLock {
+ public:
+  ~DisplayListObserverLock();
+
+ private:
+  friend class DisplayList;
+
+  explicit DisplayListObserverLock(DisplayList* display_list);
+
+  DisplayList* display_list_;
+
+  DISALLOW_COPY_AND_ASSIGN(DisplayListObserverLock);
+};
+
 // Maintains an ordered list of display::Displays as well as operations to add,
 // remove and update said list. Additionally maintains display::DisplayObservers
 // and updates them as appropriate.
@@ -43,6 +60,11 @@ class DISPLAY_EXPORT DisplayList {
 
   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 id based on display.id() as well as whether the Display
   // is the primary display.
   void UpdateDisplay(const display::Display& display, Type type);
@@ -53,11 +75,25 @@ class DISPLAY_EXPORT DisplayList {
   // Removes the Display with the specified id.
   void RemoveDisplay(int64_t id);
 
+  base::ObserverList<display::DisplayObserver>* observers() {
+    return &observers_;
+  }
+
  private:
+  friend class DisplayListObserverLock;
+
+  bool should_notify_observers() const {
+    return observer_suspend_lock_count_ == 0;
+  }
+  void IncrementObserverSuspendLockCount();
+  void DecrementObserverSuspendLockCount();
+
   std::vector<display::Display> displays_;
   int primary_display_index_ = -1;
   base::ObserverList<display::DisplayObserver> observers_;
 
+  int observer_suspend_lock_count_ = 0;
+
   DISALLOW_COPY_AND_ASSIGN(DisplayList);
 };