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Currently when we do taskbar migration, we do it for every profile
launched. After the first profile, this should be a noop, but we still
iterate over the shortcuts. Moving migration to
ChromeBrowserMainPartsWin::PostBrowserStart() will make us do it
only once.

Bug: 1132578
Change-Id: I07cb8ea74145407f978efcd6067523d7d79ca1df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2433427
Commit-Queue: David Bienvenu <davidbienvenu@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813774}