Change GetAllDisplays() in all screens to return a const vector&.

ui/display/mac/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/display/screen.h"
 
 #import <ApplicationServices/ApplicationServices.h>
 #import <Cocoa/Cocoa.h>
 #include <stdint.h>
 
@@ skipping 123 matching lines @@
     return false;
   }
 
   gfx::NativeWindow GetWindowAtScreenPoint(const gfx::Point& point) override {
     NOTIMPLEMENTED();
     return gfx::NativeWindow();
   }
 
   int GetNumDisplays() const override { return GetAllDisplays().size(); }
 
-  std::vector<Display> GetAllDisplays() const override {
+  const std::vector<Display>& GetAllDisplays() const override {
     return displays_;
   }
 
   Display GetDisplayNearestWindow(gfx::NativeView view) const override {
     NSWindow* window = nil;
 #if !defined(USE_AURA)
     if (view)
       window = [view window];
 #endif
     if (!window)
@@ skipping 143 matching lines @@
 
 }  // namespace
 
 #if !defined(USE_AURA)
 Screen* CreateNativeScreen() {
   return new ScreenMac;
 }
 #endif
 
 }  // namespace display