Make PrefService available in *ash

This CL adds a temporary solution to make the active user's pref
service available in *ash.

BUG=713934

Review-Url: https://codereview.chromium.org/2827193004
Cr-Commit-Position: refs/heads/master@{#467576}
diff --git a/ash/shell/shell_delegate_impl.cc b/ash/shell/shell_delegate_impl.cc
index 6474dd5f..dae311b 100644
--- a/ash/shell/shell_delegate_impl.cc
+++ b/ash/shell/shell_delegate_impl.cc
@@ -167,6 +167,10 @@
   return gfx::Image();
 }
 
+PrefService* ShellDelegateImpl::GetActiveUserPrefService() const {
+  return nullptr;
+}
+
 bool ShellDelegateImpl::IsTouchscreenEnabledInPrefs(
     bool use_local_state) const {
   return true;