Add EventSourceInit

The spec[1] says the EventSource constructor should take EventSourceInit
dictionary as an argument. This CL doesn't change behavior. Tests under
http/tests/eventsource cover this change.

[1] http://www.w3.org/TR/eventsource/#eventsource

BUG=403150

Review URL: https://codereview.chromium.org/561533002

git-svn-id: svn://svn.chromium.org/blink/trunk@181705 bbb929c8-8fbe-4397-9dbb-9b2b20218538