GitLab

Add methods for to get and set dictionary, time and list extension
specific ExtensionPrefs.

Design Doc: http://go/extprefrefactor

Bug: 1069560
Change-Id: I4afa211b9eb1a15f87797cb3cea6e20da9fe47ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2314992
Commit-Queue: Archana Simha <archanasimha@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791964}