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The mac implementation was hidden behind `defined(MAC_VIEWS_BROWSER)`
instead of `BUILDFLAG(MAC_VIEWS_BROWSER)` and hence was never enabled.

Since BUILDFLAG(MAC_VIEWS_BROWSER) is now always true, just remove
the check.

Bug: 849020
Change-Id: Ib4e9217304363d2d6e5ace4f1d44271ed6de1bfe
Reviewed-on: https://chromium-review.googlesource.com/c/1258027Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596078}