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This change:
1) Adds support to run-swarmed for targeting macOS
2) Adds a --swarming-os argument that allows specifying the OS the
   targets should run on, which is useful for targeting specific OS
   versions and such
3) Makes the outdir and target arguments mandatory positional args
   instead of named args, since they were/are required anyway (the
   script crashes if they aren't given)
4) Removes Fuchsia-specific defaults and documentation references
5) Renames the test_name argument to target_name for more consistency
   with gn/ninja parlance
6) Updates doc references, especially for #3

Bug: None
Change-Id: I64051d3e35d6b143d7688083a3c0877ffbf06ff3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787587
Auto-Submit: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694321}