Setting host without port should keep old port

This makes us compatible with Firefox, IE and the spec:

http://url.spec.whatwg.org/#dom-url-host

BUG=304946
R=abarth

Review URL: https://codereview.chromium.org/26506003

git-svn-id: svn://svn.chromium.org/blink/trunk@159239 bbb929c8-8fbe-4397-9dbb-9b2b20218538