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

ui/display/screen_base.cc

 // Copyright 2016 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.
 
 // This has to be before any other includes, else default is picked up.
 // See base/logging for details on this.
 #define NOTIMPLEMENTED_POLICY 5
 
 #include "ui/display/screen_base.h"
 
@@ skipping 33 matching lines @@
 }
 
 Display ScreenBase::GetDisplayNearestPoint(const gfx::Point& point) const {
   return *FindDisplayNearestPoint(display_list_.displays(), point);
 }
 
 int ScreenBase::GetNumDisplays() const {
   return static_cast<int>(display_list_.displays().size());
 }
 
-std::vector<Display> ScreenBase::GetAllDisplays() const {
+const std::vector<Display>& ScreenBase::GetAllDisplays() const {
   return display_list_.displays();
 }
 
 Display ScreenBase::GetDisplayMatching(const gfx::Rect& match_rect) const {
   const Display* match =
       FindDisplayWithBiggestIntersection(display_list_.displays(), match_rect);
   return match ? *match : GetPrimaryDisplay();
 }
 
 void ScreenBase::AddObserver(DisplayObserver* observer) {
@@ skipping 12 matching lines @@
                              is_primary ? DisplayList::Type::PRIMARY
                                         : DisplayList::Type::NOT_PRIMARY);
     return;
   }
   display_list_.UpdateDisplay(
       changed_display,
       is_primary ? DisplayList::Type::PRIMARY : DisplayList::Type::NOT_PRIMARY);
 }
 
 }  // namespace display