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

ui/views/widget/desktop_aura/desktop_screen_x11.cc

 // 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/views/widget/desktop_aura/desktop_screen_x11.h"
 
 #include <X11/extensions/Xrandr.h>
 #include <X11/Xlib.h>
 
 // It clashes with out RootWindow.
@@ skipping 147 matching lines @@
     const gfx::Point& point) {
   X11TopmostWindowFinder finder;
   return finder.FindLocalProcessWindowAt(
       DIPToPixelPoint(point), std::set<aura::Window*>());
 }
 
 int DesktopScreenX11::GetNumDisplays() const {
   return displays_.size();
 }
 
-std::vector<display::Display> DesktopScreenX11::GetAllDisplays() const {
+const std::vector<display::Display>& DesktopScreenX11::GetAllDisplays() const {
   return displays_;
 }
 
 display::Display DesktopScreenX11::GetDisplayNearestWindow(
     gfx::NativeView window) const {
   if (!window)
     return GetPrimaryDisplay();
 
   // Getting screen bounds here safely is hard.
   //
@@ skipping 224 matching lines @@
       GetPrimaryDisplay().device_scale_factor());
 }
 
 ////////////////////////////////////////////////////////////////////////////////
 
 display::Screen* CreateDesktopScreen() {
   return new DesktopScreenX11;
 }
 
 }  // namespace views