Fix PrefHashFilter's behavior when clearing a tracked pref

When a tracked pref is cleared, it will no longer be present in the
DictionaryValue that represents the pref store contents, but
FilterUpdate() and FilterSerializeData() will still be called. Make
the code handle nullptr Values safely.

Bug: 867337
Change-Id: I1840c3f60bd815145a0f765adf5907a35bca1c87
Reviewed-on: https://chromium-review.googlesource.com/1150034
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: proberge <proberge@chromium.org>
Commit-Queue: proberge <proberge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585055}
3 files changed