Set MessageEvent.source to the newly created port for shared workers' connect events

Set MessageEvent.source to the newly created port for shared workers' connect events
instead of previously null, as per the latest specification:
http://www.whatwg.org/specs/web-apps/current-work/multipage/comms.html#dom-messageevent-source
http://www.whatwg.org/specs/web-apps/current-work/multipage/workers.html#dom-sharedworker

BUG=275289
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/23319002

git-svn-id: svn://svn.chromium.org/blink/trunk@156385 bbb929c8-8fbe-4397-9dbb-9b2b20218538