Use WebPrescientNetworking::prefetchDNS instead of prefetchHostName()

This is a follow up to r150942, to obsolete prefetchDNS() API in platform API.
As WebPrescientNetworking::prefetchDNS implementation is already landed at r204938 in Chromium, it should now be safe to use.

BUG=None

Review URL: https://chromiumcodereview.appspot.com/17807002

git-svn-id: svn://svn.chromium.org/blink/trunk@153315 bbb929c8-8fbe-4397-9dbb-9b2b20218538