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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: Id816469ab29e0458a36085655a5922a3dcb3e5bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2565637Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Commit-Queue: Mattias Nissler <mnissler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832033}