Make PrefService available in *ash

ash/shell/shell_delegate_impl.cc

Index: ash/shell/shell_delegate_impl.cc
diff --git a/ash/shell/shell_delegate_impl.cc b/ash/shell/shell_delegate_impl.cc
index 6474dd5f4e74e8f7c0895faeb1eb952c7ca7ac63..dae311bd9c2ff19d88df1eb787391a751dc066a3 100644
--- a/ash/shell/shell_delegate_impl.cc
+++ b/ash/shell/shell_delegate_impl.cc
@@ -167,6 +167,10 @@ gfx::Image ShellDelegateImpl::GetDeprecatedAcceleratorImage() const {
   return gfx::Image();
 }
 
+PrefService* ShellDelegateImpl::GetActiveUserPrefService() const {
+  return nullptr;
+}
+
 bool ShellDelegateImpl::IsTouchscreenEnabledInPrefs(
     bool use_local_state) const {
   return true;