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There's no per-platform difference for this function, so consolidate
the two copies into one in first_run.cc.

No functional changes - this is just cleanup in preparation for a
first-run change for the go/lacros project.

Bug: 1129954
Test: existing unit_tests
Change-Id: I9871f2501f87fdd88f2e7598d0d43cf14fef59ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2418570
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808973}