[ServiceWorker] Make Response class better conformance with the spec.

In current implementation Response class only supports status, statusText, and headers attributes.
This change introduce type, url attributes and change the class of headers attribute from HeaderMap to Headers and make all attributes readonly.
And also this change implements the logic which is defined in the spec.

http://fetch.spec.whatwg.org/#response-class 
http://fetch.spec.whatwg.org/#response


This cl depends on http://crrev.com/329853012 and http://crrev.com/370733002

BUG=373120

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=177793

Review URL: https://codereview.chromium.org/373613004

git-svn-id: svn://svn.chromium.org/blink/trunk@177817 bbb929c8-8fbe-4397-9dbb-9b2b20218538