Use isolated storage for sign-in profile policy-installed extensions

In preparation for using custom trust anchors in the context of
policy-installed sign-in profile extensions, use isolated storage for
sign-in profile policy-installed extensions so each sign-in profile
policy-installed extension gets its own StoragePartition.
This is necessary because StoragePartition is the boundary at which
applying trust anchors makes sense, and the additional trust anchors
should not leak into other contexts.

This change does not apply to component extensions on the sign-in
screen to minimize the risk of regressions.

Bug: 939344
Test: Tests still pass, manual with planned sign-in extension use cases
Change-Id: I26131a2a96c4e0a2374f398f9dd8f2dd3a0b86e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1702405
Commit-Queue: Pavol Marko <pmarko@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Alexander Hendrich <hendrich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684695}