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This is part of the process described in r783671. This change:
1) Adds an alias from installer::InitialPreferences to the existing
   installer::MasterPreferences
2) Migrates the chrome first run code to use that new alias

Next steps are migrating every other client of MasterPreferences to use
the new alias, then renaming the class itself.

Bug: 1097204
Change-Id: I8c2f5d5a61aa127f116a255dbc2727b2f027559e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2300600Reviewed-by: Greg Thompson <grt@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790092}