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It's now redundant with is_mac.

The biggest use of it was that it set the buildflag MAC_VIEWS_BROWSER,
but I removed all references to that in the last few days:
https://cs.chromium.org/chromium/src/ui/base/BUILD.gn?q=%5CbMAC_VIEWS_BROWSER+case:yes

Bug: 832676
Change-Id: I9bd74e26a35ddb4dfec786e4b0a6ada24366c951
Reviewed-on: https://chromium-review.googlesource.com/c/1258295Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596238}