[sync] DualLayerUserPrefStore: fix clearing prefs with dots in name

Bug: 1446256
Change-Id: I67125480485cf20f834ad38c67aa7540a8ec54fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4543441


Commit-Queue: Ankush Singh <ankushkush@google.com>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1146479}