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This is the first stage in a larger refactor, which adds support for
better typing in prefs while allowing extension prefs to remain agnostic.
This CL implements the getter and setter methods for profile wide
ExtensionPrefs with string, integer and boolean values.

Design Doc: http://go/extprefrefactor

Bug: 1069560
Change-Id: I6b8901bd1856d9afbadf358229377fc2db665cfc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2276497
Commit-Queue: Archana Simha <archanasimha@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789139}