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

ash/display/screen_ash.h

Index: ash/display/screen_ash.h
diff --git a/ash/display/screen_ash.h b/ash/display/screen_ash.h
index c315caf77a55ab31ecd022241bed6c470d38c99f..4da38776f65508dfd8ce14a06611d82b075f6b3b 100644
--- a/ash/display/screen_ash.h
+++ b/ash/display/screen_ash.h
@@ -34,7 +34,7 @@ class ASH_EXPORT ScreenAsh : public display::Screen {
   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;