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This adds a new feature param (and corresponding about:flags entry)
which, when set to true, makes the profile store the default one for
new users.

Bug: 1160655
Change-Id: I549b624fdc4cf3f01fdfb0425cc12299d58ed9aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2599529
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842060}