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Up until now, HttpResponseHeaders::IsKeepAlive() only looked at the
first value of the Connection header. Since for a WebSocket response a
header like

Connection: Upgrade, close

is valid we should check other values past the first value.

BUG=371759
TEST=net_unittests

Review URL: https://codereview.chromium.org/640213002

Cr-Commit-Position: refs/heads/master@{#300087}