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The OS is now called macOS by Apple, so update the
references in the comments and documentation.

Bug: none
Change-Id: Ib4565d6f2a1c218b4efc2118385d4124c10062ea
Reviewed-on: https://chromium-review.googlesource.com/675406
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504668}