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Excluding USE_OPENSSL is no longer appropriate now that Mac is on OpenSSL/BoringSSL.
(It should have been USE_OPENSSL_CERTS.) Switch them to condition on OS instead to
match the condition in browser_options.js.

BUG=417241
TEST=Manage certificates button in Settings opens Keychain on Mac

Review URL: https://codereview.chromium.org/597913004

Cr-Commit-Position: refs/heads/master@{#296527}