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

ui/views/widget/desktop_aura/desktop_screen_x11.h

 // Copyright (c) 2013 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_VIEWS_WIDGET_DESKTOP_AURA_DESKTOP_SCREEN_X11_H_
 #define UI_VIEWS_WIDGET_DESKTOP_AURA_DESKTOP_SCREEN_X11_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 23 matching lines @@
  public:
   DesktopScreenX11();
 
   ~DesktopScreenX11() override;
 
   // Overridden from display::Screen:
   gfx::Point GetCursorScreenPoint() override;
   bool IsWindowUnderCursor(gfx::NativeWindow window) override;
   gfx::NativeWindow GetWindowAtScreenPoint(const gfx::Point& point) override;
   int GetNumDisplays() const override;
-  std::vector<display::Display> GetAllDisplays() const override;
+  const std::vector<display::Display>& GetAllDisplays() const override;
   display::Display GetDisplayNearestWindow(
       gfx::NativeView window) const override;
   display::Display GetDisplayNearestPoint(
       const gfx::Point& point) const override;
   display::Display GetDisplayMatching(
       const gfx::Rect& match_rect) const override;
   display::Display GetPrimaryDisplay() const override;
   void AddObserver(display::DisplayObserver* observer) override;
   void RemoveObserver(display::DisplayObserver* observer) override;
 
@@ skipping 43 matching lines @@
   display::DisplayChangeNotifier change_notifier_;
 
   ui::X11AtomCache atom_cache_;
 
   DISALLOW_COPY_AND_ASSIGN(DesktopScreenX11);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_WIDGET_DESKTOP_AURA_DESKTOP_SCREEN_X11_H_