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Host resolver might return address list that contains the same address twice or more.  In this case, we should not process the address for websocket throttling multiple times.
If we don't have duplicate check of address, it would put the job throttle by the job itself (deadlock).

I think this is cause of failure in Linux perf (webkit.org) bot and websocket fail to connect localhost on ubuntu (crbug.com/36652)

BUG=36652,41319,40995
TEST=none

Review URL: http://codereview.chromium.org/1750001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@45156 0039d316-1c4b-4281-b951-d872f2087c98