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Async DNS is fairly stable at the moment, so we don't really need the
toggle in about:flags anymore. (Note that the --enable-async-dns and
--disable-async-dns command-line flags will still work for now.)

BUG=432236

Review URL: https://codereview.chromium.org/840933003

Cr-Commit-Position: refs/heads/master@{#310552}