Destroy NightLightController before PrefService

ash/shell.cc

Index: ash/shell.cc
diff --git a/ash/shell.cc b/ash/shell.cc
index 72fac6a55ab24a15cab0393890a9edc6d0f2c3a1..754d5e5c9774bfbfbf4a77bfe35ead20a50844ed 100644
--- a/ash/shell.cc
+++ b/ash/shell.cc
@@ -785,6 +785,9 @@ Shell::~Shell() {
   shell_port_.reset();
   session_controller_->RemoveObserver(this);
   wallpaper_delegate_.reset();
+  // NightLightController depeneds on the PrefService and must be destructed
+  // before it. crbug.com/724231.
+  night_light_controller_ = nullptr;
   pref_service_ = nullptr;
   shell_delegate_.reset();