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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}