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This is a reland of 4e425ce4

Original change's description:
> mac: introduce system infobar
> 
> This infobar will be used (via field trials) to display updated info
> about current OS support and provide users with a link to more info.
> The infobar receives both activation info (which configs to show for)
> and the actual strings to display via field trials, so it can be updated
> for deployed Chrome instances without an update cycle.
> 
> This change adds:
> * The infobar itself
> * A feature MacSystemInfobar that controls whether the feature is
>   enabled and if so which configs (Arm or Rosetta) it's enabled in
> * A switch --force-mac-system-infobar which causes the infobar to act as
>   though it was running on a specified system, for both manual and
>   automatic tests
> * A variety of IDS_MAC_SYSTEM_INFOBAR_* constants to customize the
>   infobar's message, link, and link text
> * Unit tests for the logic to interpret the feature and the
>   enable/disable configs
> 
> Bug: 1116578
> Change-Id: Ie7fd6e772a0112a17e794d1b8fcb009c57ebed4a
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2360479
> Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
> Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
> Reviewed-by: Peter Kasting <pkasting@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#799615}

Bug: 1116578
Change-Id: Ie98bc3a18fc4f952819b3ee40a1c49953e8c63ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2364893
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800317}