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This CL implements the getter and setter methods for extension specific
ExtensionPrefs with string, integer and boolean values.

Design Doc: http://go/extprefrefactor

Bug: 1069560
Change-Id: I238cde708b678deb18ac0b5d97641c0be8bfe14c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2300326
Commit-Queue: Archana Simha <archanasimha@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790573}