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Background: All our app registry read/write access permissions
are enforced by the compiler. We used `friend` declarations before.

Introduce WebAppRegistrarMutable subtype for WebAppRegistrar.

While WebAppRegistrar serves as read-only model, WebAppRegistrarMutable
is a "view" which allows writable access for an owner which has
WebAppRegistrarMutable pointer.

WebAppSyncBridge is the only controller which has that writable access.

Bug: 860583
Change-Id: I4623ce826d25f0c2bbf9ed685a932d7eb734bdcd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1820497
Commit-Queue: Alexey Baskakov <loyso@chromium.org>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699248}