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This patch moves this free function from generic net_util.h header,
into url_util.h, because it operates on a URL part (hostname), so it
seems more appropriate there than in the soon-to-go-way net_util.h.

BUG=488531
TEST=net_unittests --gtest_filter=UrlUtil*
R=eroman@chromium.org
TBR=felt@chromium.org

Review URL: https://codereview.chromium.org/1629733002

Cr-Commit-Position: refs/heads/master@{#371768}