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

ash/display/screen_ash.h

 // Copyright 2014 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 ASH_DISPLAY_SCREEN_ASH_H_
 #define ASH_DISPLAY_SCREEN_ASH_H_
 
 #include <stdint.h>
 
 #include "ash/ash_export.h"
@@ skipping 16 matching lines @@
 class ASH_EXPORT ScreenAsh : public display::Screen {
  public:
   ScreenAsh();
   ~ScreenAsh() override;
 
   // display::Screen overrides:
   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 view) 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;
 
   // CreateDisplayManager with a ScreenAsh instance.
   static display::DisplayManager* CreateDisplayManager();
 
   // Create a screen instance to be used during shutdown.
   static void CreateScreenForShutdown();
 
  private:
   DISALLOW_COPY_AND_ASSIGN(ScreenAsh);
 };
 
 }  // namespace ash
 
 #endif  // ASH_DISPLAY_SCREEN_ASH_H_