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This unifies using user type filtering for extension based and web
default apps via json file declaration.

DD: https://goto.google.com/web_app_default

webapp support was added here: crrev.com/c/1401508

crrev.com/i/784837 - update default app list for all boards. These
are only apps that has child_users in external_extensions.json
and no managed_users are used currently in external_extensions.json

crrev.com/c/1407458 - update script that generates default extensions.

TEST=Locally, set of apps as expected on nocturne for different user
types. Unit test added
BUG=921121

Change-Id: I655c1e3bf1e9974aed52e0b74d544a7d41450214
Reviewed-on: https://chromium-review.googlesource.com/c/1407498Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Commit-Queue: Yury Khmel <khmel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625784}