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This is preparatory to adding a new kDefaultInitialPrefs, which will be
"initial_preferences", and adding support for reading preferences from
that file in addition to the legacy file.

Bug: 1097209
Change-Id: I35508175274451171eb6bd5fa10a528de7bdfe1f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2555563
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830708}