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This new device setting will supersede the existing "startup flags"
setting. The latter was storing chrome://flags feature flags as raw
command line switches, which turned out to be problematic since Chrome
can't easily map the switches back to feature flags to validate them.
Subsequent changes will add support for the new way of doing things as
well as migration logic in session_manager and Chrome.

BUG=chromium:1073940
TEST=None

Change-Id: I54204286db5262e6ccc76b844db5863e69c1d8c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2290730
Commit-Queue: Mattias Nissler <mnissler@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#831789}