Make PrefService available in *ash

ash/mus/shell_delegate_mus.cc

Index: ash/mus/shell_delegate_mus.cc
diff --git a/ash/mus/shell_delegate_mus.cc b/ash/mus/shell_delegate_mus.cc
index f0932ce7e5b20185ee9ac052a39283b9c0774a87..6e7bd139ca3f140c2eddd9e53a49c48d2523b5c8 100644
--- a/ash/mus/shell_delegate_mus.cc
+++ b/ash/mus/shell_delegate_mus.cc
@@ -155,6 +155,14 @@ gfx::Image ShellDelegateMus::GetDeprecatedAcceleratorImage() const {
   return gfx::Image();
 }
 
+PrefService* ShellDelegateMus::GetActiveUserPrefService() const {
+  // This code should never be called in the case of Config::MASH. Rather, the
+  // PrefService instance is stored by Shell when it manages to connect to the
+  // pref service in Chrome.
+  NOTREACHED();
+  return nullptr;
+}
+
 bool ShellDelegateMus::IsTouchscreenEnabledInPrefs(bool use_local_state) const {
   NOTIMPLEMENTED();
   return true;