Add metrics for the number of attached displays.

ui/gfx/screen_mac.mm

 // Copyright (c) 2012 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.
 
 #include "ui/gfx/screen.h"
 
 #import <ApplicationServices/ApplicationServices.h>
 #import <Cocoa/Cocoa.h>
 
 #include <map>
@@ skipping 73 matching lines @@
 
 class ScreenMac : public gfx::Screen {
  public:
   ScreenMac() {
     displays_ = BuildDisplaysFromQuartz();
 
     CGDisplayRegisterReconfigurationCallback(
         ScreenMac::DisplayReconfigurationCallBack, this);
   }
 
+  virtual ~ScreenMac() {
+    CGDisplayRemoveReconfigurationCallback(
+        ScreenMac::DisplayReconfigurationCallBack, this);
+  }
+
   gfx::Point GetCursorScreenPoint() override {
     NSPoint mouseLocation  = [NSEvent mouseLocation];
     // Flip coordinates to gfx (0,0 in top-left corner) using primary screen.
     NSScreen* screen = [[NSScreen screens] objectAtIndex:0];
     mouseLocation.y = NSMaxY([screen frame]) - mouseLocation.y;
     return gfx::Point(mouseLocation.x, mouseLocation.y);
   }
 
   gfx::NativeWindow GetWindowUnderCursor() override {
     NOTIMPLEMENTED();
@@ skipping 160 matching lines @@
 
 namespace gfx {
 
 #if !defined(USE_AURA)
 Screen* CreateNativeScreen() {
   return new ScreenMac;
 }
 #endif
 
 }