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

ash/display/screen_ash.cc

Index: ash/display/screen_ash.cc
diff --git a/ash/display/screen_ash.cc b/ash/display/screen_ash.cc
index bc1a81cf7da8eba43d3cbaf24536d33255005cc2..70dd5a82a7b02bb5c836806983dba350c396ce27 100644
--- a/ash/display/screen_ash.cc
+++ b/ash/display/screen_ash.cc
@@ -45,7 +45,7 @@ class ScreenForShutdown : public display::Screen {
     return NULL;
   }
   int GetNumDisplays() const override { return display_list_.size(); }
-  std::vector<display::Display> GetAllDisplays() const override {
+  const std::vector<display::Display>& GetAllDisplays() const override {
     return display_list_;
   }
   display::Display GetDisplayNearestWindow(
@@ -110,7 +110,7 @@ int ScreenAsh::GetNumDisplays() const {
   return GetDisplayManager()->GetNumDisplays();
 }
 
-std::vector<display::Display> ScreenAsh::GetAllDisplays() const {
+const std::vector<display::Display>& ScreenAsh::GetAllDisplays() const {
   return GetDisplayManager()->active_display_list();
 }